How to flatten photographic cellophane?

First, cut the size a little bigger and remove the lining on the back.

wet the sticker and the glass with a wet rag.

then paste it, taking care not to leave air. If there are bubbles, just uncover them and stick them on.

wipe it evenly with a rag. After all the water is drained, cut off the redundant places on the four sides.

I tried it three times, and the effect was good twice. There was bubble once, because it was a piece of glass that I couldn't take off, so I couldn't stick it well.