These instructions should not be attempted on an actual rock face as there is a difference in procedure between an indoor rock gym and a rock face.
Step 1: Prepare for climbing
First double up both of your webbings (if they are too long), so that it reaches from your harness's belay loop to slightly above your head. Then attach a locking carabiner to one of the webbings and then to the belay loop of your harness. Lock the carabiner. Then use another locking carabiner at the other end of the webbing which will be placed on an accessory loop of the harness for now. Do the same with the other webbing. Hook the ATC to a locking carabiner and place it on an accessory loop, and keep your extra carabiner on an accessory loop. You can also bring an extra ATC with you as a backup if you have it. (I can't tell you how many times I have accidentally dropped equipment when rappelling!)
You can also use extra cords to ensure that you will not drop equipment, however, these can get tangled and in the way very easily.