Learning regulator service is an incremental process for most people and a great regulator to start with is the US Divers Aquarius. Easy to find, Easy to service, Reliable and Inexpensive.
Local dive shop discard pile, Craigslist, E-bay, Swap Meet, Flea Market etc.... I have seen Aquarius regulators in all of those places at practically give away prices.
You need basic hand tools, an IP gauge to hook on any BCD hose and a bit of patience and time. This is a great regulator to begin and sharpen your skills on.
Below are links to the Service Manual, Repair kit for both the first and second stage and a link to the IP gauge.
Aquarius First and Second Stage Procedures
Aquarius First Stage Service Kit
Aquarius Second Stage Service Kit
Intermediate Pressure Gauge
The second stage on the Aquarius is the same as found on all US Divers regulators from the late 60's through the early 80's. Conshelf, Calypso etc. Makes a great safe 2nd when diving your double hose....IP gauge can be used on the hookah port of your DA or Royal with adaptor or on any Phoenix or Argonaut.