The Gummi ship creation system is complex and deep, (more the former than the latter, sadly), so I can't really do it complete justice in this space. Suffice it to say that you can pretty much just get by with the blueprints and blocks you find over the course of a normal playthrough. I found it to be moderately entertaining to customize my ship, but it has very little impact on gameplay except for making the Gummi ship flying sections easier. There's a FAQ over on GameFAQs you might want to read, although the data there is more about the blocks and some general advice than any sort of tutorial.
To access the Gummi ship interface, you can either press Square on the "universe map" screen, or you can talk to Cid in Traverse town:
From what I recall, he's usually in the Accessory Shop once you have access to this feature. He'll eventually sell gummi blocks, in case the sets you find over the course of the game are not adequate to satisfy your creative urges.
The editor itself is a tangled mess. Expect a high learning curve. There's a tutorial that is terribly inadequate, and you can pull up the controls, for what that helps. The problems with it boil down to trying to navigate an editing reticule in 3D space with a bunch of 2D controls and a 2D display. Sometimes you'll think you're right on top of a block, and you're not - you're on a different block, or you're off the ship altogether.
When it comes to ship building, you're generally trying to optimize across several axes:
- The armor of your ship, which increases its ability to take damage and survive
- The speed of your ship, which is a combination of engine power and wing maneuverability, and helps you avoid enemies and enemy fire
- The firepower of your ship, which is controlled by its guns
There are also a few special items which have special powers, such as the cockpit's ability to heal damage, or the Haste gummi's ability to give you an afterburner power. The blocks themselves are all named after Final Fantasy series spells.
When it comes to building ships, the only requirements are:
- One cockpit
- Engine (or engines) pointing towards the rear of the design
This makes a ship blueprint "flyable." Connectedness of the blocks is largely irrelevant, from what I recall. You can build and find non-flyable ships, but you won't be able to use them to play the rail shooter Gummi ship minigame.
There is also a constraint on the number of blocks in the design and the overall size of the build area, but there are upgrades Cid sells at certain plot milestones which can help you grow these limits.
I generally just built a giant flying brick with wings on the side and a metric ton of guns and other useful bits hanging off of it. Sure, it wasn't ascetically pleasing, but nothing really survived long enough to complain about it.