Usually on pop ups that spin will take it back towards the field, but sometimes it will hug the fence - be ready for both.
   
Look for arm motion from pitcher as to pop-up direction. Turn around to face the fence And look up.

Take the mask off but hold it until you have located the ball and can see which way it is drifting, then drop the mask away from you on the other side.

Backpedal to stay ahead of the drift, because the spin will likely take the ball to the field.  Catch with mitt and bare hand (alligator style) and cradle the mitt to your chest to hold on.
 

First feel for the fence with your throwing arm.
If there is a warning track and you reach it, drop to a figure-4 baserunner slide, lead foot to the fence, and slip under the ball - better than crashing into it. (Before the game, count the running strides from catcher's box to backstop both straight back and towards on deck circles.)