Welcome back! You get masks, pelts, and antlers by having other deer cast them on you. The only ones you can get by yourself are red pelt (by running through the crying idol) and the white pelt (by praying to the Twin Gods). You can also pick up flowers on your antlers if you walk through a patch of them (look for a new button in your action bar). Try hanging out with a big group of deer and I'm sure you'll get the hang of it, it is very fun to figure out!