Drop in the image Letterboxd makes when you tap Share → Instagram Stories. The poster and your star rating are kept, the backdrop is removed, and you get a transparent PNG to use as a sticker.
Poster boxes are kept whole with rounded corners. Avatar boxes keep a circle, for your profile picture. Cut-out boxes keep only what stands out from the backdrop, like stars and text. Drag on empty space to add a box, tap ⇄ to change its type, and drag a box or its corners to adjust it.