Whilst I'm not a professional sub builder and I haven't uploaded anything to the workshop yet, I'd like to help out the community by providing a basic guide to the sub editor so people can, at the very least, make easy modifications to other subs or make basic structures.
Warnings & disclaimers: I am by no means a professional, and everything in this guide is subject to change as the game is still in early access. In addition, making a submarine is very taxing & is often times not the most enjoyable or fun thing to do. Lastly, I highly suggest you don't make cheaty submarines in order to remove the fun & challenge of the game.
Other Barotrauma Guides:
The Easy Parts
The Barotrauma submarine editor isn't very user friendly, and even most workshop uploaders haven't offered or provided much insight on how to use it. It's certainly something you need to experiment with & figure out on your own.
The controls are pretty simple and much like the normal game, it's mostly just figuring out where to put what and how to make things work together. The DEL key on your keyboard is used to delete objects, and you can use WASD and the mouse to zoom in & out and navigate the area. Not sure what else to put here, so I'll conclude it here.
Once you enter the sub editor, all you see is a blue backdrop and some menus around the borders of the game. At the top, you have your file controls & the submarine placer. The submarine placer allows you to place the outline of a submarine into the editor - AND WHEN YOU LOAD THE SUB IN-GAME, IT WILL SPAWN THE ACTUAL SUB - And also connect it to a docking hatch aboard the actual sub. Then you have the Back button, the Open button, and the Save button. Using open, you can access all your current .sub files and load them in. Editing a submarine you've opened does not change the sub file, unless you save it over the original sub file. If you accidentally overwrite a vanilla sub, you can back it up using the file verification system under Properties. You can easily make a modified version of a vanilla sub if you open the .sub file, make your modifications, and then save it under a different name. This preserves the default sub that comes with the game or the original file, but also makes a new .sub file for you to use.
There is also wiring mode. You need to know how to wire stuff & how wiring works. I'm not sure if there's a tutorial on this, but it's just like if you wire something on a sub-ingame. Except you are just a floating screwdriver with infinite wires. I'm not going to add anything else here since I don't know what else to add.
You can use this mode to store items in containers. If you aren't in character mode, you have your editor box at the bottom. You can search for & place down an item in the editor, enter character mode, pick it up in character mode, and then put the item in a container or in a weapon. Character mode allows you to interact with stuff as if you were a character, it does not spawn an actual character or entity into the game. You can also wire stuff in this mode. You can probably repair stuff, but I haven't tried that yet.
The visibility tab is on the left side by default. You can toggle lighting, walls, structures, items, waypoints, spawnpoints, links, hulls, and gaps. Lighting is for any lights you have on-board and also makes the background darker & more like the actual game. Items is for things like fabricators and storage. Waypoints are only used for AI, so it's optional but will make your ship incompatible for AI. Spawnpoints are where you & your crew spawn - if you have none, you spawn next to the colony and die almost instantly from the pressure. Having multiple is recommended. Links are wires & other links, like a weapon periscope to a weapon or a junction box to a reactor.
Walls are the backgrounds. They have no collision and aren't even necessary to prevent water from coming in, but highly recommended for the interior of a sub. Structures are things like the floor, ceiling, doors, and the shell of the submarine/shuttle. Hulls are what mark the interior of the shuttle - If there is a hull, water will not be there. Hulls are also for individual rooms, you do not want one hull for the whole sub, as this will mean one little leak floods the whole thing. Gaps are for separating hulls & allow air & water to go through. Most doors and hatches automatically generate gaps, though. I'm not exactly sure how the game handles water going through a gap, but having a hatch usually stops it.
The Complicated Part
This is the hard part, and where I can't explain everything. Most of the stuff here is on you and up to you to figure out.
Shells are the exterior of the ship. They have collision and players can't go through them. Hulls aren't needed in the space between a room and a shell. The game has multiple shells available, but currently only 2 variants are available - Shuttle shells and Shell A. Each variant has multiple & varying degrees. The most helpful advice I can offer for placing shells is mirroring them. By this, I mean if you click on the shell, you will see a bunch of options & data come up. Near the bottom of the interface are some buttons, labeled "Mirror X", "Mirror Y", and some others. "Mirror Y" will be the most helpful for creating the underside of the ship. Finally, most shells or shell-objects (such as chains or tailfins) have been designed for specific vanilla ships. However, the Shell-A is used for all ships and should be able to create most things. Just don't try to make any 90 degree angles, it won't work out well.
Walls are slightly difficult to place and their scaling is a bit weird. You can edit the texture scaling by clicking on them, though, if you want to make some adjustments. I can't explain how to place walls correctly as it's a bit confusing & wonky, but not hard to figure out. Walls have no effect on gameplay, but they are pretty much essential if you want your sub to look like a sub.
Inside walls are walls that have collision and players can walk on (and, if you have a plasma cutter, cut through.) These are how you make rooms. There aren't very many variations of wall, but you'll manage. There isn't very much to them.