Since I had to go about rebuilding my brake MC, and I love knowing absolutely everything about something before diving in to fix it, I took some pictures and documented everything.
What you need:
Flat head screwdriver
Phillips head screwdriver
5(mm?) allen wrench
longish circlip pliers
Swiss army knife pick
what is super helpful:
syringe from vet store or flavor injector
garbage bag to cover bike from brake fluid
At first I thought I could do this with the MC on the bike. If you have before, you probably can, but it is soooo much easier off the bike.
Rip a hole in a garbage bag and slide the mirror and right controls through it.
Take off brake lever with 10mm socket on the bottom and flat head screwdriver on top.
Open the reservoir and use syringe to draw out all the fluid. Replace cap.
Use 12mm socket to remove the banjo bolt going into the MC. Careful! There is a very thin washer that may stick to the MC. Hold on to it.
Use the allen wrench to remove the mirror/MC assembly.
Use the pick to get under the rubber dust cover near where the brake lever was. It may take a little work (don't rip it) since it can be weathered.
Use circlip pliers to nab that circlip.
Slide out piston and spring.
Look at the layout of everything in front of you including the order. No surprises that way going back in.
Looks simple enough. Rubber with dimple mates to a hole on the spring. Put that in the MC. Use the swiss pick to get the rubber ring on the piston. You have to stretch it a lot, but it can take it. Just don't use the point and make sure it is facing the same direction as the one you took off. Hold in piston and put on circlip. Dust boot back on. Voila! You dun it! Toss it back on and bleed the brakes.