In general, emergency oxygen kits for divers are very simple and easy to use by design. However, to properly use and maintain any kit you should take an Emergency O2 Provider course offered by all major dive training agencies. Most courses are completed in a single evening and many courses offer the option of CPR certification as well. Kit maintenance is similar to periodic maintenance for your scuba tanks and regulators: the oxygen cylinder will need a hydro test and visual inspection every five years (but no need for an annual VIP as you should never immerse a medical oxygen cylinder) and the oxygen regulator should be serviced at that time as well.
The cylinder in the kits are always shipped empty, so you will need to arrange to get an oxygen fill in your emergency oxygen cylinder. Local practices vary depending on the gas supplier, but in general the US Federal Food and Drug Adminstration (FDA) rules allow local compressed gas suppliers to fill medical oxygen cylinders with USP grade oxygen without a prescription when the cylinder is labeled "OXYGEN Emergency Use Only". Your Emergency O2 Provider training course will include specifics on how best to get emergency oxygen fills in your area.