Once you load in game, not much will change if you don't have a custom rig.

However, if you do, then you'll see some new options inside the dressing room!
There, you will get a preview of your custom rig, along with some new buttons on the control panels.

The Dressing Room

The details on the side show how optimized your avatar is. It's not a full system, but it gives you an idea of what's going on. It shows your vertex count, your material count, and your textures count.

Big textures (>8192) will show a warning and give you an automatic bad rating, along with expensive shader functions and some other things.

On the other side, you'll notice two new buttons. An upload button, and a refresh button.
The refresh button, well, refreshes your avatar when clicked, in case anything goes wonky. The upload button is a bit more complicated:

The upload button

For other players to see your avatar, you have to upload it to the server. Pressing this button will upload your current bundle so other players can see it when you join a park or fight them in a match! There is a progress bar in gym to show how far along it is.

Server Status

This simply shows how up to date your current avatar file is. If it says Not Uploaded in big scary red, you should probably upload it if you want others to see the updated version!

If it says Up to Date, however, you're good.

Helpp I can't edit my in-game outfit

If you were able to change your outfit with an avatar on, it'd replace it and probably break everything. Just turn it off in the mean time.

What about others?

There's not much you have to do specifically about this one, but I'll explain it anyways!
When you load into any room (such as a park or match), it will try and load everyone's avatars, at least, the people who have them.

The amount that can happen at once is specified by the Concurrent Downloads setting (see below), but eventually all will be loaded, at least the one's below the Max MB Download limit. You can see who's avatar is currently loading by a ghost-like material.
The specific visibility of one's avatar is determined by numerous settings (also see below), all merged into one visibility target.

Toggle for Others will turn everyone else off no matter what else you have toggled.
If that's on, but they have their avatar hidden for others, then it'll hide.
Similar to above, if in a match, it'll hide their avatar if they have that turned off as well.
Last in the priority is toggle if newer version, which hides people's avatars with a higher version of the mod than you do.

That weird icon you can see sometimes

That's an icon who simply show's who has the mod or not. If it's red, that means your avatar is not visible to them with their mix of settings. Or they just have a grudge on you specifically.

ModUI Toggles

• Reload Keybind: A keybind (typed in words) that corresponds to the key that resets your avatar when clicked. This is by default the R key. The reload keybind uses key names for the key, not the literal key itself. For example, ";" would be "Semicolon", and "\" would be Blackslash.
• Reload button: Just a button that reloads your current avatar upon toggling.

Avatar Visibility

• Toggle for Self: Toggles whether you see your custom avatar locally. This does not affect what other players see.
• Toggle for Others: Toggles whether you can see other players' custom avatars.
• Let Others See My Avatar: Controls whether other players can see your custom avatar. This setting is networked.
• Toggle In Match: Toggles whether or not you and other players can see your custom avatar in a match. This setting in networked.
• Toggle If Newer Version: Toggles people's avatars who have a higher version of the mod than you, to prevent any forwards-compatibility issues.
• Toggle In Rock Cam: Toggles whether your Custom Avatar shows up in Rock Cam or not. This only applies upon reloading your avatar.

Statistics

• Log Avatar Statistics (self): If enabled, logs mesh info like vertex count, material count, etc. when the local player's avatar is loaded.
• Log Avatar Statistics (others): If enabled, logs mesh info like vertex count, material count, etc. when a remote player's avatar is loaded.

Download & Upload

• Max File Download Size: The max download size for other avatars in MB.
• Max Concurrent Downloads: The maximum number of downloads that can be ran at the same time
• Upload Avatar: Uploads the avatar in the folder when the button is clicked and saved.