Tie one end of the rope around yourself using a bowline. leaving a 1-2meter tail. Clip the carabiner in. Pass the sheet end around the tree. Make sure that it is as close to the ground as possible. Italian hitch the sheet end to the carabiner. This will serve as a brake. Descend with caution.

The 1-2 meter tail is for you to tie yourself off with if the descent is more than the length of rope available.

Remember that this is an inherently dangerous maneuver. If there is an alternative take it.