What others have described mirrors my technique but I lay masking tape the full length of the mylar span-wise and crease it sharply at the leading mylar edge using the "handle" of a single-edge razor blade to form the hinge. I do not sand either surface but surgically clean both, first with acetone and then alcohol and clean, cotton cloth. I then never touch the bonding surfaces. The red Tesa transfer tape is best, in my experience. Fresh transfer tape and mylar is preferred. Pre-applied mylar/transfer tape has not worked for me.

Removal is a PITA. The transfer tape may come off with the mylar if it is not ancient and your heart is pure. Be careful not to peel up paint. If not, I then try rolling it up with fingers. If these fail, then solvents and potentially a gooey mess is next. 3M general adhesive remover is best, imo. Get the liquid in a can and place it in a squeeze condiment bottle. I cut paper towels into strips and lay it on the offending tape. Soak with solvent and wait. If is a warm day, I cover it with plastic qrap to slow evaporation. I shear up the softened transfer tape with a chisel-shaped tongue depressor.

It is possible that the trailing edge of the wing is not span-wise straight.. It is also possible that the mylar will not lie straight span-wise. There is essentially zero "give" in the mylar. Let it lie where it wants and masking tape the leading edge down. An extra pair of hands is valuable. Don't touch the bonding surface or let them touch anything else post-cleaning.

Post-Application: The ship and materials need to be warm (70-80 degrees) and kept warm and unloaded post-application for a day or two (more is better). I keep mylar seals unloaded in the trailer by using control locks. I have seen 3 brand-new gliders experience 100% top surface mylar debond since the controls lay in the trailers during shipment at full deflection, keeping the them loaded.