We typically put the tape on the bottom.... I think more from an appearance reason than anything else. (can't see the tape when the plane is sitting on the ground unless you are looking for it.)

Deflect the surface fully away from the side you are puttng the tape on
stretch the tape out and adhere it to the fixed surface.
Carefully tuck it into the hinge line and smooth it all down.

Some tapes you can apply windex to the covering to allow the tape to slide... then "squeegee" the windex out fom under the tape. This helps you to not have to fight wrinkles in the tape.

You can also use Monokote or other iron-on covering to do the same thing.... technique is similar, just add the heat at the correct places at the correct times.

Sealing the gap can improve aileron precision near center and can help prevent the cntrol surfaces from fluttering. The tape also can act as a sort of back-up to the hinges... and hide a failed hinge. Inspect the hinges carefully on a regular basis.