I do this all the time, it's straight forward except for these points.
- Be careful of the tiny screws, they can fall into the camera and that is a bit of a disaster, you can use needle nosed tweezers to lift them out and to place them back into the countersunk screw holes or magnetized screw drivers.
- The metal tabs need to be lifted gently, I use a flat jewelery screwdriver.
- Oily hands are a not recommended so wash them or wear lint free gloves.
- Copper shims are very thin and bend easily, so be careful, they have notches on the bottom to align them, so if you take them out, taken note of the orientation and the order they are in.
- Take note of the orientation of the SI screen, if you put it in back to front, you'll loose the red AF indicators. This is also has orientating tabs on it's bottom side. Don't force anything, it all fits together perfectly, you may need to align things a few times to get it right, everything should just slip into place.
- Can't warn you enough how fragile the SI screen is so use a blower or wet clean with a sensor swab and solution only.
- You will need to blow on both sides of the focusing and SI screens.
- Some dust maybe on the bottom of the pentaprism block, so this will require a static brush, dust aid or senor swab, I wouldn't use a blower in this area as the dust can be pushed further into the viewfinder assembly.
This is another good guide from Shimworldhttp://shimworld.wordpress.com …-the-5d-mk-ii-viewfinder/
I'd do this in steps, if you happy once you clean the focusing screen, then don't be tempted to go further and clean the SI screen and pentaprism block, as Bohdank says you can easily introduce more dust into the viewfinder.