Once you know what you’re doing with the shape, it’s time to draw it up and get it in the press. Unless you want to get fancy and layer up your own base, your best bet is to use a single sheet of plywood, between 5-10mm depending on your board shape. Lighter is better obviously, but if your board is going to be massive you’ll want something a bit more substantial.
Then simply measure up and draw out your design on the ply before cutting it out, preferably with a jigsaw for accuracy. Then you’ll need to get it in a press to get that rockered shape else you’ll be frontfliping your way down the mountain.
Again it’s up to you exactly where it’ll go; do you want a flat base between the bindings and a spooned nose or more of a banana shape? As Bud Keane would say, it’s up to you man…
Your best bet here is to use either a bench or pallet and nail/screw some sort of ply-bending device together; it doesn’t have to be pretty, it just needs to hold together for a couple of days. Wood bends better when wet so you can wrap it up in damp towels, or even better steam it if you have access to a sauna/hot tub. Don’t soak it as it’ll deaden, just keep it damp. And keep it in place for at least 48hrs so the shape will hold.