Here’s a series of pics of how I do it.
Tethered in....
Rappel rope attached to tree above the tether....
Run rappel line thru ATC (ensuring the rope goes around the carabiner) and clip autoblock carabiner into the lineman’s loop. Once you’ve checked to ensure the proper setup transfer your weight from the tether to the rappel setup.....
Now you should be looking like this.....
Attach your pull down rope to the rappel rope....
Place your left hand on the rope above your ATC and your right hand on the autoblock prussic. Applying pressure to the top of the prussic will loosen it and will allow you to start going down. If you’re not moving then you have the tag end pulled straight down and it’s locking in the ATC (which is how it’s designed to work) and you’ll need to pull the rope away from your body a bit to descend.
Practice with this at ground level until you get a feel for how the set up reacts. Once you have a feel for it go up one stick high and practice from there.
When you’re down, pull on the pull down rope to retrieve your rappel rope....