You don't join PRS and MCPS, that's for the composers and the organisations then collect royalties on their behalf.

If you're recording someone else's music, you must inform MCPS and they calculate the fee, based on what proportion of the album is copyright material, the selling price and how many units you'll be manufacturing. There's a flat-rate fee if you're making less than 1000, but for small numbers or where most of the material isn't copyright then you may be better off calculating the fee. Full details are on their website.

Retail Audio Products: AP1 and AP2 Licences

The system isn't really geared up for small-scale, private recordings probably issued on CD-R, but the legal position is the same and you should follow it. Even if all the material is non-copyright you should still get clearance from MCPS first who will confirm that no royalties are payable.

For live performances, PRS will charge a fee to the venue - as a performer you don't have to worry about it. You may be asked to complete a return with details of your set, but this usually applies only to the larger venues.

If the composer isn't a member of PRS/MCPS then strictly speaking you're supposed to seek them out and agree a fee privately. If they're a mate then that may be practical. Otherwise, in the folk world at least it's possibly safe to assume that if someone hasn't joined then they're not concerned about protecting their copyright. However that's no defence if their lawyers come after you!