back to kel-tec stuff
first get a small screwdriver or punch to push through the center of the bottom of the mag, then push the original bottom forward and off the mag, be careful as the bottom plate will be under spring tension, and the spring will fly out if you don't hold it in.
put on the l shaped bottom plate so that the bottom of the plate is behind the rear of the mag. then slide the new +1 extension to the rear until it locks into the bottom plate.
some +1 extensions have been reported to be flat which lead to problems with the mag seating into the pistol. newer ones (i think anyway) have a bevel on the l shaped piece (red arrow) so that it will not catch on the hammer spring catch (part 279) when the mag is inserted.
if you are having problems with the l piece hitting the hammer spring catch, you can bevel the corner of the catch (red arrow) to allow it to clear and be easily inserted.
some have had problems with the p-3at and the +1 extension not being able to be inserted into the pistol with a full 7 rounds because the top round can not be pushed down at all into the mag (and it must be able to be pushed down a bit to be inserted into the pistol).
if you look at the +1 extension from the end and the top you can see that the bottom "ledge" (red arrow) sticks out into middle more than the little top "ear"
the long side of the bottom of the follower can catch on this larger bottom ledge and not allow the follower to enter the +1 extension. you can either bevel the lower ledge of the +1 extension, or file down the longer edge of the follower a little (it should not take much).