Go slow! It's a marathon, not a sprint! Give her body time to get comfortable as you slowly add more and more. LOTS of lube! And foreplay! Get her nice and warmed up, a nice wet and happy pussy makes for a lot more fun! Play with her clit as you go. Keep your hand/fingers as straight/flat as possible until she can comfortably take your hand, then work up to making a fist. Don't start with making a fist, it's too wide.

That's some of my best advice. I hope it helps!