To actually rappel out of the tree here is the process.
1. Girth hitch your rappel rope to the tree.
2. Attach your descender/carabiner to the rappel rope.
3. Attach your descender to your bridge/harness.
4. Slowly descend.
5. Once you're on the ground, you pull your yellow retrieval line to get your rappelling rope down.
The only tricky part is stopping to retrieve your platform. That's where the mechanical descender, like the Madrock Lifeguard, comes in handy. Previously I was using the figure 8 descender and I had to tie off (like in the video above). It worked fine, but it was a little nerve wracking. With the mechanical descender, you still have to tie off, but that is simply a backup to the mechanics.
Sent from my Galaxy S8.