Well it takes a month from when you registered your pictogram for your fawn to grow up into an adult. Then you have to get someone to cast spells on you.

Certain trees can be rubbed that drop pinecones. You eat those to cast antler spells.
There are other trees with mushrooms growing on them. Eat them to cast mask spells.
And to get pelt spells, simply sit next to a sleeping deer.

Just remember you cannot cast spells on yourself, so you need to get someone else to cast them for you.