I'm assuming standard 38mm curved Mylar for ailerons etc.

1.Mark a line using a sharp chinagraph pencil and STRAIGHT edge where the
leading edge of the Mylar needs to be - make sure you get this bit right.I
use a steel straight edge.

2.Lay the Mylar out and tack it in place using the line as a reference and
small pieces of masking tape.Then when you're happy,run a full length
piece of masking tape along the edge.This then positions the Mylar.Do this
as many times as it takes to get it straight and in the right place.

3.With the Mylar still just laying under its own weight on the wing/tail
surface,remove the pink adhesive strip backing tape at 45 degrees away
from under the Mylar.Do this slowly,a bit at a time, and just touch it to
the surface as you move along pulling the pink strip away.

4.When the whole strip is tacked in position you can go back along it and
roll the adhesive strip down.

5.Use white Tesa capping tape to finish it off.Always cut small 45 degree
corners off tapes then the corner point won't get snagged and start to
peel - a round curve is even nicer if you have time and/or care!

TIPS.

1.Ensure the surface is perfectly clean with no lumps that could cause the
Mylar to go away from true.The cleaning part takes most of the time but is
very important.

2.NEVER force the mylar - the curve in it gives it a natural straight
lay.There IS a small amount of tolerance but it IS small.

3. Take your time - you only have to do this once every several
years.Allow at least 2 hours per control surface.

4.Some gliders have a recess of about 10 thou for the adhesive part of the
tape to sit in - don't assume it's straight - don't ask!

5.Enlist a friend who's as keen as you to see that it gets done right -
you may need another pair of hands although I managed alone using this
method.Worked ok on my Libelle which didn't have recesses for the aileron
Mylar, and Discus which did.

GOOD LUCK