The smogon Create a Pokemon project isn't as simple as you making a design and using it, it's a large community driven project where dozens of people work on the design, function and mechanics of a Pokemon over the course of many months.
I'm not sure how dated the post is but you can use this as a starting point.
The only other way to create your own Pokemon and use them in a Showdown server is to be able to program the specifics of the Pokemon and add it to your own server. That is to say, to use existing or create brand new assets and code and either have someone implement it into their server or for you to create your own and have no restrictions.
So join the smogon project or start learning and getting familiar with code and learn to do it yourself.
