You need to show you meet the full sea time: Three years total service on oceans or near coastal, with at least two years of this total as Mate or Master, and at least one year one year of that mate/master time on vessels over 50 GRT. Sea time used for other endorsements can be “re-used” for this. If you have the time, you take a test or a course approved to substitute for the Coast Guard test.
STCW is more complicated. Depends on what STCW you already have. Too many scenarios to attempt to explain without more information.