Here I show the repair of a detached celluloid pickguard on an older Martin acoustic guitar. 

This one is a challenging repair - looks easy, but it can miss and that would be a shame. 

Celluloid unfortunately has the property that it shrinks over time. To the best of my knowledge, Martin glued the beater boards wet to the lacquer. The celluloid and the nitro lacquer used form a hard and only conditionally flexible bond. If the pickguard shrinks, high forces occur - where either the bonding or one of the two partners gives up. Quite typical damage patterns are then that the pickguard bursts away or worse, cracks appear on the ceiling next to the glued seam. In this case, the bonding had come apart and only very small cracks had formed in the wood.

With a spatula, a little heat from a hair dryer and caution, you can get the rest off the blanket.

The old pickguard is glued permanently with spray adhesive. To apply this is taped around the paint surface cleanly and with newspaper the ceiling protected from splashes.

A slightly curled pickguard can be repaired with a few small clamps and a shim. It is best to leave it overnight, clean and polish it and it will look like new!