To make a balanced map, you have to start out with the basics, which is the layout itself, the Slasher should not spawn too close to the survivors (See: Construct) but should also not spawn too far away from the survivors (See: Vyten).
The best examples of perfect spawn points can be found on Summercamp, Highschool, Asylum and Shambles.
If you've got your layout done, you can move on to the next step.

Now that you've got your Slasher and Survivor spawn points, we need to talk about miscellaneous spawns, like Generators, Helicopters, etc.

Regular items such as the Milk Jug or whatever, should virtually spawn in any location of the map that would actually make sense for these items to be placed at, such as tables, desks, counters, benches, you get the gist.

Gas Cans should be placed anywhere that isn't "too close" to the Generator, things like lockers, shelves, etc. work just fine.

Batteries should ALWAYS be placed close to Generators.

Generators should be placed in any highly detailed part of the map, or a memorable one at that. (See: Shambles having Generators mostly spawn behind the detailed houses)

Helicopters are the same deal, place them at memorable parts of the map, this could even be the survivors' spawn (See: Hospital). Just make sure there's enough room for it to land, otherwise the Helicopter's blades will clip through the brushes and it'll just look wack.

Now that you've got your own map done, (or ported a pre-existing one) with a nice layout for
everything, the easiest part of this guide can begin.

If you've made your own map, you can skip to Part 4 of this guide.

If your map is a pre-existing one from the gmod workshop, you might have to tune it up a little bit to actually make it playable for SlashCo.
Adding things like extra rooms or areas that the players can access (See: Highschool with its Basement and Rooftop)
Or alternatively editing the map so that there's no stuck spots, (See: Shambles with its million ladders).
No matter what map you try to port to SlashCo, it's always gonna need to be tuned up just to prevent funny things from happening.
And don't forget, your map NEEDS a Navmesh, otherwise slashers like Male07, Criminal and Free Smiley Dealer will not work properly.

Welcome to the most time-consuming part of making a SlashCo map!
What you want to do for this part is get a text editor that isn't just Notepad, so things like Notepad++ or Sublime Text will work.
Now what you want to do is make a new lua file, and call it your map name, so in my case it was "sc_shambles.lua".
Once you've done that, you need to specify where everything will spawn manually, luckily there's already a template for that, so you don't have to go through the trouble of having to rewrite everything from scratch.
The template can be found here: https://www.dropbox.com/scl/fi/53tyygwb71vbj0w7bk6kw/template.lua?rlkey=hxon8mpf3fbylqilpqzek4vmb&dl=0
Or alternatively you can decompile SlashCo and find the template in "slashco\lua\slashco\configs\maps"
Starting off with the Manifest:
https://i.imgur.com/xQiuPSV.png
"Name:" is pretty straightforward, you put the name of your map in there, ("SlashCo Flatgrass"), etc.
"Author", if your map is a port from the gmod workshop, you should include the original creator too.
"Default", just keep it at true.
"Size", you should not set the value to a random number, as it actually affects certain gameplay elements, such as Tyler's manifestation time, etc. you can probably just eye the map out and figure out the right size to set.
"MinimumPlayers" is the amount of players needed for the map to have a chance to be chosen.
"Levels" "if your map has multiple floors/levels, set them up here. these are Z coordiantes a couple of units above the floor, around 50 or 100" - Manti

That's all there is to the Manifest, you need to handle the rest by yourself though.
If you scroll down further in the template, you'll see that there's spawnpoints defined for everything that I've mentioned previously.
You'll have to get the coordinates from the game or hammer editor.

If you want to see your coordinates in game, you have to do the following:
Open the console
Type: "cl_showpos 1"
Follow the "pos" and "ang" values, ignore "vel". "pos" stands for position and "ang" stands for angle.
https://i.imgur.com/Ge2DsH4.png

If you want to see your coordinates in hammer, then well, you can't, but you can see the coordinates of an entity that you've selected, or a brush.
Once you click on anything in Hammer, the coordinates of that entity/whatever will appear in the bottom right corner.
https://i.imgur.com/zswSHhn.png

Once you get the angle and the position of an item, generator, you name it, you'll have to enter both coordinates into the respective line in the lua file.

Now you'll have to do this for every spawn, items, generators, slashers, survivors.
You want to preferably have 8 different survivor spawns, a few slashers spawns (if you care about the nightmare offering), somewhere around 100 gas can spawns, 8 generator spawns, 4-8 battery spawns, 75-100 item spawns, and 3-4 helicopter spawns.
All of these can vary depending on map size, sometimes you might need more item spawns, sometimes less, same with everything else.
Once you've filled out your lua file with all the necessary details, you can finally begin to pack your map.
You MUST remember to do this, otherwise your map won't ever show up!
You need to include your lua file in your mod folder with the map, with the following path: "lua\slashco\configs\maps\"
https://i.imgur.com/sSsjGui.png

Once you've playtested your map, you can send it off to the workshop with an appropriate thumbnail.
If you have any other questions in regards to mapping for SlashCo, you can join the Discord server here: https://discord.gg/g8HsZ4qxdY
Or add me on Steam for further questions.