You may have heard of Hammer in the mapping community. J.A.C.K. is basically a better version of the Hammer used in the Half-Life SDK (Valve's modding tools for GoldSrc). We're going to be setting up and using J.A.C.K. in this guide.

You need to download J.A.C.K. first. You can get it either from Steam or https://jack.hlfx.ru/en/download.html (not sure what's up with the Windows download at the moment, but Gamabanana seems to have a few downloads of it, not sure if they're the latest versions though).

Next, you need the FGD file.
The best version of the FGD: https://twhl.info/vault/view/6501 (thanks Bright Moon!)
This is the FGD I edited originally from Valve to add a few things. https://drive.google.com/file/d/1L70knohUKqjREwTFqAPGyYnYXK4GBrZM/view?usp=sharing
This is Valve's one though (if you want it): https://developer.valvesoftware.com/wiki/Condition_Zero_Deleted_Scenes.fgd

There are Hammer/J.A.C.K. Icons here[twhl.info] that you can use instead of the default cubes to help visualise entities. (thanks Bright Moon)

One last thing, you should get the VHLT/ZHLT tools here: http://zhlt.info/

• Extract it and run it. Go to Tools > Options...
  
• Click on the "Game Profiles" tab.
  
• Under "Configuration:", click the "Edit..." button to the right.
  
• Click "Add..." and name it along the lines of "Condition-Zero" or "Deleted-Scenes".
  
• Click "OK" then "Close".
  
• In the "Game" tab, you can see "Game Data Files:". This is where the FGD comes in. Click "Add..." and select your FGD. (I saved mine in my czeror folder in Half-Life in steamapps). You can now go to the "Directories" tab (still under "Configuration Settings").
  
• Under "Game Executable", click the elipses (...) to the right and navigate to your "hl.exe" in your steamapps > Half-Life folder. Select it.
  
• Under "Base Game Directory", do the same but for the folder "Half-Life" and not the executable (e.g. my path would be C:/Program Files (x86)/Steam/steamapps/common/Half-Life).
  
• Under "Mod Directory", you select the "czeror" folder in your Half-Life folder.
  
• Under "Source Maps Directory", select the maps folder inside the czeror folder (mine would be C:/Program Files (x86)/Steam/steamapps/common/Half-Life/czeror/maps for example).
  
• Don't worry about "Model Viewer" for now.
  
• Go to the "Textures" tab. This is where the WAD (acronym for "Where's all the data?!") files are selected. WAD files hold textures to be used in-game.
  
• Click "Add Files..." and select a WAD file in the czeror folder. You can use any of these you want, and can make your own, just don't select too many as GoldSrc may have a limit. Don't worry about any of the other options here for now.
  
• Go to "Build Programs". Here is what tells J.A.C.K. how to compile your maps. Just locate and select the right files in the VHLT/ZHLT folder you saved earlier.
  
• Under "CSG Executable", select "hlcsg30_x86.exe". Do this for the next three, their names in J.A.C.K. relate to their file names (except for LIGHT, which is rad in the files).
  
• Now click "Apply" and "OK" in the window, and you're done setting it up!

Video version:

Click "File", then "New". When prompted to "Select a game", choose your "Condition-Zero" or "Deleted-Scenes" option you made earlier and click "OK".
"Default Room Properties" will then show up. This you don't need, so you can just click "Cancel". You can check the checkbox which says "Don't show this dialog again" if you don't want the window to pop up again.
Now you have 4 windows and tools on the left and other stuff on the right.

Hammer and J.A.C.K. (any Sledge but that's not being actively developed on anymore) have 4 views. The top left is the 3D view, the top right is top-down, and the two bottom views are from the sides. In the 3D view, you can hover your mouse over it, and press "z" to be able to move around like you're in noclip. WASD and mouse to look. Press "z" again to come out of it.

On the left hand side, there are the tools.

• The top one is the "Selection tool", where you can select brushes, entities, etc. and transform them and move them.
  
• The next one down is the "Magnify" tool. I've never used this before so chances are you won't use it either.
  
• Next one down is the "Camera" tool. Also never needed to use this.
  
• Next one down is the "Entity Creation Tool". This is very useful and is used to make the player's start point, enemies, lights, props/models, etc. You'll use it a lot.
  
• Next one down is the "Block Creation Tool". This is what you'll use to make brushes/blocks for the floor, walls and ceilings, as well as doors, windows, stairs, vents, triggers, etc.
  
• The next tool down is the "Texture Application Mode". This is a powerful tool as it lets you select a Face of a block and change its texture (size, location, rotation, fit-to-face, transfer texture settings from one face to another, etc.). You'll probably use this a lot too.
  
• The next tool is the "Apply Current Texture to Selection" tool. This takes the current texture selected in the Texture Browser on the right, and applies it to a block.
  
• Next tool is "Apply Decals". This just applies decals to faces. Decals are basically overlays on blocks which can give extra detail.
  
• Next tool is the "Clipping Tool". This is used to "clip" blocks into two parts. This can make ramps, cut corners, etc. It's really useful.
  
• The next tool is the "Vertex Manipulation" tool. This lets you take the vertices of a block and move them on their own. However, they have limitations. Moving vertices in certain ways can break the brushwork and cause compile errors. It's very powerful, but also hard to use because of this.
  
• The last tool is the "Path Tool". I haven't used this, but I'm pretty sure it's for when you want something to move on a path (e.g. the starting sequence in Half-Life or the "On Rails" chapter, also in Half-Life).

The main thing you need to know on the right, is the Texture Browser. This lets you browse textures to apply to block/brushes. You can search for certain textures in the "Filters" part near the bottom after clicking on the "Browse..." button.

Click the "Block Creation Tool" and drag in one of the 2D views. This makes a rectangle with a lined edge. You can drag on the corner squares in any of the 2D views to change the size of it. When you like it, you can press "Enter" to create it. Now in the 3D view, you can look at the block and see how awesome it is. You may notice it might not have the texture you want. Browse for textures, find one you want, select the block with the "Selection Tool" selected, and click the "Applt Current Texture to Selection" button. Now the block has a better texture!

To make a spawn point, click on the "Entity Creation Tool" and click on a block. You now see another block, but it's a solid colour. Click the "Selection Tool" and double click the entity. an "Object Properties" window will now appear. The entity's type is in the box under "Class". You can change the entity by changing this. Type "info_player_start" in it if it's not there already. You now have a player spawn point! Press the X in the top right of the box and it saves the settings automatically.

Before you compile your map, you need to make sure there aren't any leaks. Leaks are what happen when there's a "hole" in your map that lets light or entities escape to the void. The void is the nothingness around your map. To fix leaks, the easiest and quickest (although not the most efficient) way is to make a massive block around your entire map, and then make it hollow. To do this, drag a massive block in one of the 2D views with the "Block Creation Tool" selected, and make sure it surrounds every other block and entity in all the 2D views. Then press "Enter" to make it. Then go to Tools > Make Hollow. Use a value of -32 to hollow it outwards. Now you see a big block around your level. If you want to make it look like the sky, then click the "Browse" button int eh texture browser and search for "sky". If a sky texture does not show up, then you need to add a WAD file to J.A.C.K. I suggest adding "alamo.wad" because it includes all the textures needed for clipping, triggers, etc. Select the sky texture and apply it to the big block. You can now click the compile button (it looks like a gamepad on the top row of buttons), save your map, and then a window will appear. Set everything to normal and tick "Don't run the game". Click "OK". Now open Condition-Zero Deleted Scenes and open the console with the ` key. Type "map [yourmapname]" and then "Enter" to load your map. It looks bad because there's no lighting.

If your map is set outside, make an entity and set it's class to "light_environment". It doesn't matter where you place this entity (the light comes from the sky texture).
If you want lights indoors, the main one to use it "light" (still an entity). Place it where you want the light to come from.
Now compile and play your map and you have light!

Visit my YouTube channel for more tutorials, and here is a playlist of all of them so far: https://www.youtube.com/playlist?list=PL22A8NtuBYhGW_SWzJxWMqVmxYxfPUGAr