when you boot up the game you should see "Create Offline Game", Click that and select "Map Making Mode". Offline is pretty useful since closed or private can close your lobby when your internet cuts out :)

Selecting Map making mode helps turn on flying, some debug things, invisible blocks being seen, and placing spawns for stuff. We need this mode to help create a map and save it.

You should be dropped in your lobby.

Now click on the map (Blockshowcase) on the top left. After you clicked that, go to Create a new map.

Before you generate a new map, you can change a few things.

change the size to how you want it by clicking on the width, depth, and height bar on the top. i'm going to change the map to the smallest it can be.

Name it! Maps can always be renamed in the end, but its good to keep a name on it so it won't be lost in your pile of NewMap000's

Floor type is the most important thing to generating a map. I'm going to change it to Grass, but you can change it to what you want your players to walk on.

Fill free to mess with terrain type and hills, but i would select the terrain to flat.

Generate the map!

You should see a bird's eye view of your map in the background of your lobby, if it's nothing how you wanted it to be then generate a new map to fit your needs.

start the game and select the minergun and sniper (i'll tell you why later)

Once you're on your map, change to the minergun and start building!

the controls for miner gun are easy, the buttons will be on the screen but use your arrow keys/dpad for navigating.

here are the block placing options and what they do:

Face: What Face does is place blocks on the side of other blocks.

Float: Place a block in mid-air or replace blocks that are in it's square bubble.

Fill: Place it on a side of a block and watch it spread like cup of water being knocked over onto the floor. (Press Alt to change the fill mode, experiment with it)

Line: The most useful option for block placing in my opinion, using the line option will shoot a line of blocks to the direction of where you place your block. it will stop if a block is in it's way.

Switch to your sniper. This is a useful technique i use to make some maps.
While holding the sniper, press Alt, on the bottom right you should see Destroy and Create radius.

change the block on your minergun and hover over it, fire your sniper on create radius and you should get a simple circle block of the block you chose on the minergun.

once you have your map built, you need to do a few more things.

Press escape and select Map options.

Spawn Settings: clicking on here will bring you to show spawn zones and place spawns at your current location, There is another guide on murder miners guides to learn spawning and i recommend you go there.

Set camera: Setting the camera will replace the bird's eye view background to what you see as you click on the option.

Murder the flag options: setting the flags for murder the flag at your current position.

Weapon Spawns: Placing weapons at where your position is.

Vehicle Spawns: Placing the Murdercycle and Murdercopter to your current position.

Skybox settings: click on it to change the skybox.

Ambience Type: Select what sound you want playing in the background of your map, useful for atmosphere.

Fall looping: be able to fall off the map and be placed randomly in the sky to fall onto the map again.

Lock floor, ceiling, and wall blocks: This will make blocks indestructible on the flooring, walls, and ceiling on the map. (note it has to reach to the end where you can't place blocks beyond the point)

Locked blocksMake certain blocks placeable and indestructible by selecting them.

You can save your map on the bottom, or click on "Save as" to change the name of the map.

Once you finish your map and save it, you can select it in "my maps".

I apologize for describing half the things in this guide by telling you to experiment it yourself and having no screenshots whatsoever.

hopefully you learn how to make a map for others to enjoy.