I just bought a MIM P-Bass and I want to replace the pickups with Seymour Duncan 1/4 pounders.

How easy is this to do? I've never replaced pickups before. Is there an online guide somewhere that I can follow or do the pickups come with instructions?

Any advice would be useful.
All you have to do is take off your pickguard. There will be 2 wires running from your pickups onto your volume pot, usually a red and black. Unsolder them, then solder your new corresponding wires into the same spot.