how to fit a sapphire crystal

Professional installation is strongly recommended, unless you are an experienced amateur/enthusiast with the correct tools you should not attempt it under any circumstances. Sapphire crystals can crack if care is not taken, particularly if they are not introduced perfectly square during the pressing-in process. The crystal gasket may also be easily damaged if care is not taken rendering it unusable. High domed bubble crystals can be notoriously tricky to press in squarely if not supported with a suitable mandrel. So unless you have the tools and experience please leave it to the professionals.