Skins and board trails are purchased with cosmetic currency.
All cosmetics in Bleeding Edge are unlocked with the same cosmetic currency earned by leveling your account. Playing the game, working with your team, and achieving a sweet victory will earn you the necessary experience, but leveling takes time. At the time of writing this guide, I'm at Account Rank 8 and right next to 2500 Currencies, and some of the best skins go for a Thousand Cosmetic Currencies or more.
You'll want to prioritize the cosmetics you want if you want to unlock them at a constant rate. Once you have the necessary cosmetic currency, you can purchase new skins and board build tracks through the Workshop tab in the main menu. Once inside, choose a character who can use paintings, then navigate to the paintings submenu. Unlocking boards is universal for all characters, so you don't need to worry about buying a skin multiple times (you just want to choose a character that can climb boards, as the menu won't be accessible otherwise ).
There are skins, alternate paint schemes, stickers, and tracks up for grabs. Customizing your board in Bleeding Edge can get expensive, so know what you want before you jump into a spending spree.
Once in a match, you can climb onto your board (or generally for characters who don't use the board) at any time by pressing right on the D-Pad or Z. If you get hit while channeling your mount , you will stop, so try and do it away from enemies. Most attacks won't knock you off your mount once you mount it, but any crowd control will.