Good resource that was incredibly useful to me when I first dabbled with the sub editor:
https://barotrauma.gamepedia.com/User:J ... r_Tutorial
Also a similar thread with some design tips:
viewtopic.php?f=18&t=7539
In my experience it takes a surprisingly long time to make a sub, and the bigger/more complex the sub the longer it can take but even a small and simple sub is probably close to 2 hours of "work". The process I've used to minimize rework:
1) sketch out the sub layout on some paper, brainstorm what I'm going for
2) build the walls/wallpaper
3) Add "hulls", doors/hatches, and gaps (doors will already generate their own gaps)
4) Add all the "stuff" such as machines (generator, navigation console, pumps, etc), buttons, spawnpoints, player equipment
5) Electrical - wiring it all together. It's important to do this last because it's the pain in the ass to fix wiring after you move a bunch of stuff
6) Once you've got the ship together, load it up in single player and test it. See how the lighting is, that everything works etc. Then tweak it more in the editor and test it again in single player. Go back an forth like this until you're able to complete missions solo with it and feel it's finished.
Hope that helps - happy to answer any questions you come across as you're figuring out the editor. Btw you can also load up other subs in the editor which can be great for dissecting them and learning how they are made.