This assumes a hinged bezel, if it is a snap on fit remove it in similar fashion or if it is a screw on type screw if off then just ignore the bits related to the hinged type.
It also assumes an acrylic crystal, a glass crystal can me fitted in a similar way but do not try bending it!
First open the bezel using a knife at 1 o’clock if it hinges at 6 o’clock or opposite the hinge if it is hinged elsewhere. Be careful of the hands! Work over a plain surface such as a piece of writing paper so that if you should knock one off you will be able to find it.
Seat the crystal into its groove at 6 o’clock, then work round both sides together pushing it into the grove, have your thumbs underneath pressing with your fingers on top.
As you get close to the finish you may need to apply pressure upwards towards the centre of the crystal with your thumbs whilst pressing down with your fingers to bow the crystal up in the centre and down at the edge, this should let you seat the crystal into its groove so that when you release the pressure it is held in firmly.
Old glass crystals are usually not perfectly round soif it is a little loose try rotating it in the bezel until it grips.
If it becomes necessary to glue the crystal in than the correct type of glue must be used so that crystal can subsequently be removed and any surplus removed relatively easily - which should be done when the glue if partially set.
Before closing or refitting the bezel check that you have not displaced the hands, particularly that the hour hand clears the second hand (or the second hand clears the minute hand on a watch with a centre sweep second hand) and that the minute hand clears the hour hand.