First off I would HIGHLY recommend you get a copy of the SiG Armorer's DVD. You can find it at such vendors as TopGunSupply. Invaluable to any SIG owner, really. Then DO NOT REMOVE the roll pin that holds the bottom of the mainspring assembly onto the frame while it is in it's pre-disassembly position (on the frame).
If you are impatient an wish to do the work yourself sooner rather than later, then at least watch one of Todd's videos on YouTube. He does a pretty good job with them and he works for Cusomized Creations, a refinishing outfit that also does customized gunsmithing. Here is one on the P6 (a Police issue variant of the P225): YouTube - AA Todd McKechnie Customized Creationz Sig Sauer P6 P 6 P225 225 Disassembly Video Part 1 3-09
Once you have the mainspring assembly off the frame, you can then tap out the roll pin, carefully, slide off the old spring and then replace the spring and put the roll pin back. Look carefully at the roll pin as they generally go in AND out from the same side and usually it's the left.
Shouldn't be any need for special tools, just a little time and some patience.