As all the others players said, either buy Procurer/Covetor BPC and invent the Hulk/Skiff BPC or just buy the Hulk/Skiff BPC also by contracts.
Most of the time, those contracts are seeded in commercial hubs like Jita or Amarr.
If you join a bigger corporation/alliance, most of them have at least one player owning the Covetor/procurer BPO who can easily print you copies.
Once you have those Hulk/Skiff BPC in hands, just take into account that this is not an easy industrial jobs. You need moon goo, make reactions to have the basic elements like Sylmaric Fibers and Crystalline Carbonite for exemple. With those, you must build more advanced components like ion thrusters and Pulse shield emitter. And only when you will have the required amount of those advanced elements, you will build your hulk or skiff.
so it takes times, it takes skills, it takes assets (reaction formula, advanced elements BPC). And you must have stations with good rigs in LS/NS to make it profitable. When you combine all of those, you can be profitable with a very good margin. It’s not that sure in a non rigged station in HS for exemple.
Most of the time, you will find it is cheaper and faster to buy your hulk/skiff directly. But it can be a player project and a satisfaction to build your own skiff/hulk, no matter the cost. It’s a game after all… Only you can decide.