Position your vinyl sheet on the substrate, centering the letter/graphics, and securing it with masking tape. The tape will act as a hinge for the project. You will be able to see the letter/graphics through the semi-transparent transfer tape (top layer).
Flip the vinyl sheet upward and slowly peel off the release liner (bottom layer). The lettering/graphics should now rest on the transfer tape (top layer). If the lettering/graphics wants to stick to the release liner, slowly manipulate the liner to help the letters/graphics stick to the transfer tape.
Let the transfer tape with vinyl letters/graphics fold back down into position, holding the edge so it does not actually touch the substrate until you are ready. Using a squeegee, or some kind of hard but gentle edge (i.e. plastic spatula, CD jewel case or large tongue depressor), firmly rub over the transfer tape to secure the letters/graphics to the substrate. Work from the center out to each side.
Note: If you have a vinyl sheet too large to handle in one hand, after it is hinged in place, you can cut between letters/graphics at intervals more easily manipulated at one time.
Then, carefully peel off the transfer tape to reveal your lettering, now adhered to your substrate. If any part of the vinyl comes up with the transfer tape, lay it back into place, rub it down more and retry lifting transfer tape off.
Removal: When it comes time to remove your vinyl letters/graphics, use a blow dryer on high heat, one letter at a time, to soften vinyl and make it easier to peel off.