Whoops, forgot to answer. That was why I quoted you in the first place.
You find blueprints in the environment. For example, you might find a noisemaker which takes two ethanol, a charge pack and an injector. You highlight the item by holding circle (or equivalent), hit L1 and it brings up the crafting screen. You commit items you find around the environment. Then, when you've committed everything needed to craft something, you hit L1 again and you craft the item, which costs you some scrap at that point.
You nearly always have more scrap than the 100 limit, and you frequently find more supplies, so always be crafting basically.
A little extra aside: you can commit materials to items even if you're already at the item maxed limit, you just can't actually hit L1 and seal the deal. This lets you carry extra materials later in the game.