Since she is so young, I would put something sweet on there like molasses or one of those apple strips and let her bit herself for the first few times. Just hold it and let her lick it and if she opens her mouth, just gently slide it in to where it should be. Leave it there for a moment then gently slide it out (make sure you don't bump her teeth. I would then put some more sweet on it and let her lick it again before calling it a day. This way, she will associate the bit with good things.
Or, you can put a bit/bridle on her (like you would with any other horse) over a halter then hand graze her at a particularly grassy spot for a while (with the bit in).
Just always associate the bit with good things and bitting will never be a problem. ;)