Adding your first ship

Short overview of what you need to do until guide get refined more assuming you already have sprite:

Create entry in ship_data.csv (copy vanilla file and clean up everything, could leave one entry as base for you ship)
Create corresponding .ship file - preferably using one of ship editors (see Community Modding Tools)
If everything right your ship now real, could spawn it with console commands.
Now you can go fancy and create .variant for it (also preferably using ship editors)
If you have variant you can add it to default_ship_roles.json. Now if faction knows ship it will spawn in its fleets.