How do I make a developer product?

I read the wiki for roblox but I still don't understand it.

Check the blog, it has a section on it that's quite detailed.

You go to your place, from develop, go to configure, go on developer product, make one, and in your game, look up in free models developer product button or make your own and edit it to your approval, grab the ID of the dev product (which can be seen after you make it), and then when you visit your game you should be able to buy it, but also you need scripts to award the product.