Do a search online for some DIY
guide or video.
Here is a thread with a
guide but these calipers are different to yours - but fundamentally pretty much the same process.
Not sure exactly how the Brembo's are made up but a lot of calipers use a 7mm Hex head pin - needing a 7mm allen key or box adapter for a ratchet, do some research and see if yours has 7mm hex head parts and then check you have one in your tool kit. Dont do what I did and assume your extensive allen key set covers 7mm as mine covered upto 6mm then jumped to 8 then 10, so I had to abandon mine halfway through and go get a 7mm one from a motorfactors.
http://www.seatcupra.net/forums/showthread.php?t=140322
Make sure when you free the caliper from its bracket that you have a way of taking teh strain off the brake hose, I use long tyewraps to hang it from the suspension springs, others use a correct height bucket to sit it on, but you don't want to leave it dangling and supported only by its brake hose.
The retaining bolts holding the caliper to its bracket will be big and probably tight so you will need a fair bit of force to loosen them.
You can use copper grease however most places recommend using a brake grease instead. You need to be extremely careful with grease as clearly it would not be good if it gets on the braking surface of the pad or disc. If you do use grease, a thin smear on the metal back of the pad and a very small amount where the pad metal edge contacts the runner in the caliper. this helps reduce the posibility of squealing as well as allowing the pad to slide easier, but be very careful not to over do it. A lot of pads actually come with a peel off sticky pad on the back rather than using grease.
You also need to get some brake cleaner and de-grease the disc face (both sides) before installing the disc as depending on what sort you get, disc manufacturers either coat them with rust protective lubricant - so they dont rust while on the storage shelf, or coat them with a zinc finish - same reason. either way you want to get all the oil off the disc before fitting with brake cleaner and cloth.
Re your confusion about rear pistons. The piston is the part that the hydraulic pressure pushes out to force the pads to squeeze together, these are on all calipers, when you do your fronts you will need to push the piston back into the caliper with either a dedicated piston winding tool, or a g clamp (or if you are lucky just with force from your hands (you need to remove the top of your brake fluid reservoir and check it doesnt overflow when you push the piston back in and hence the fluid back in the system.
The difference between the front and rear caliper pistons is that the fronts should just push straight back with pressure (a tip is to leave the old pad in while doing it as it gives you a good surface to push on rather than an awkward round thin rim (you will see what I mean when you get it apart) but the backs have a thread mechanism that winds out, so to push the rear piston back in you need to apply pressure and screw it in at the same time made much easier by using a proper piston wind in tool (pretty cheap -£10ish).
Dont forget to pump up your brakes from the brake pedal before doing the other side or going for a drive, and take it steady braking for the first few hundred miles.
Good luck.