To make a skateboard with one's own hands is not so difficult. If of course we speak about a deck. it is better to buy the rest. And decks break often.
For a start we need polyfoam or more dense material. We mark and cut out edges proceeding from what inclination we want to receive.
It's important not only to cut out, but also to polish a blank. Actually it is not so difficult.
Then take thin sheets of a tree (plywood), coat with glue and put in a bag. Extort air from a bag. It is possible not to extort, but it will be not so strong. Further put plywood on our preparation, press and leave for 48 hours before full drying.