Fitting round plastic crystals
First, remove movement, band, and anything else such as a rotating divers bezel from case.
(1) Remove old crystal from bezel. (2) Remove all old glue and dirt from bezel shoulder. (3) Clean bezel and case. (4) Measure distance across bezel. (5) choose a crystal 0.10 mm larger than measured distance. (6) Apply a small amount of plastic crystal cement to bezel shoulder ( I use G-S cement). (7) Insert crystal with crystal press or crystal inserting tool. (8) Clean watch dial and hands. (9) Replace movement into case. (10) Put on case back and divers bezel, etc.
To clean dial, always use blower and a sharpened piece of pegwood or toothpick to remove dust and broken pieces of glass from surface. Then use Rodico or something like it to remove any other residue. (Rodico is like silly putty and can be purchased from most material distributors.)
It is always best to remove the hands when cleaning the dial or to straighten them.
To straighten hands, use sharpened pegwood and tweezers, hold hand steady by inserting pegwood through the hole in the hand and use tweezers to manipulate hand. You can also use a hand or balance tack to hold hand steady.
When choosing a replacement crystal consider the design of the watch, the height of the original, the style of the original (high dome, low dome, flat top, stepped, tension ring, straight sided, magnifier, etc.).