Really easy:
1) Climb on the arch of your foot, this will give you the most traction. To do this turn your feet like the Flinstones.
2) Pull yourself with your hands against the tree
3) Mimic the sloth
4) Do not be scared or look down, you may lose your balance and that will hurt. Which brings me to my last tip -
5) Do not fall

Here is a little video of how I do it:

Hang around until I post some tips on how to get down.