You basically push against the caliper (which retracts the piston) using a screw driver as a lever against the caliper carrier.
I suck at explaining but it's extremely simple. I have a track day this coming weekend so I'll take pics or make a quick video clip when I swap pads.
As for the tool TC Kline posted: I have one I use on small calipers. I'm 99% sure it wouldn't work on the M3's large front calipers.