Bill,
How do you attach it to your harness? Make sure you try it hanging from a couple of feet off the ground so you know you can reach the cam lever to allow yourself to be descended down the tree. The more you use it the more comfortable you'll become with it.
I find that I attach my harness to my life line at the base of the tree and then but my harness on because the GriGri is right behind my neck. When I come down, I just pull all the rope out so I'm free of the GriGri at the bottom.
Good Luck...Jeff