How do you replace the avatar with a sphere or mesh?

I want to make a game like Gravity Shift, and the first step was to make the character a ball, or a mesh that you could unlock after beating levels. I found a forum on how to override the avatar with a model by putting a model with the character inside it into StarterPlayer and naming it StarterCharacter. I did that but instead, I put a sphere part inside of the model. I named it "Head" and made it the primary part to fix some issues in the output. It still isn't working, and nothing is appearing in the output. I have a doc with some pictures of the models in it. If you could tell me how to do it or send me the link to a published model or Roblox place, I will be grateful.

If you could help me make it roll and move, that would also be helpful, but I first want to start with step 1.