Oh that, it's quite simple. I will assume you already have already imported the texture into your model and wrapped it. If you have, you just click upload in the model section. It'll ask for a couple general information, basically actual Model, Portrait Model (Can be an alternate model instead of the portrait if you want.) ect. After all these pesky details, you go to the bottom of the page, agree to the hive submission rules. They are not long, so I suggest viewing them before you continue, since it saves you from a lot of future trouble.
Then, you'll automatically will be linked forward to enable you to attach skins. That's about it. After all skins that are custom has been attached, you'll be done and the model will be uploaded.