First, you need to decide how big a panel, and whether it is going to be freestanding or wall hanging, or pedestal mounted, etc.
Then you need to decide what service hazards it is going to be exposed to.
Then you can specify the physical panel itself.
Then you have to install the i/o and the other devices, and then you have to wire them according to code, recommended practice, and the customer's
specifications.
If you don't know how to do this, I strongly recommend that you hire a system integrator with a panel fabricating facility to do it for you.