I cleaned my focusing screen today on my 5D and it was not hard at all. I tried a technique rarely talked about online. It took less than 5 minutes and now my screen looks great! IMPORTANT: This method is only for the removal and cleaning of the focusing screen (not mirror, and not sensor) on the 5Ds, other models may vary.
1. Wash hands with warm water and dry.
2. Remove lens and gently pull (with index finger) focusing screen retaining latch. The screen bracket will drop down into a lower position for removal.
3. Using clean tweezers, grasp protruding plastic tab on the right side of focusing screen and pull it up and out of bracket.
4. Using clean fingers grasp focusing screen on two adjacent corners and rinse both sides with luke warm water. This should remove any dust or debris that is sitting on the external surfaces (front and back) of the screen.
5. Still holding onto the 2 outside edges of the focusing screen, wick off any large droplets of water using the corner edge of a piece of paper towel. Then using a hair dryer set to low blow the remaining surfaces dry holding the screen at least 12 inches from the hair dryer. The idea is to NOT heat the screen but to blow dry the 2 surfaces of the screen. This took about 2-3 minutes.
6. Still holding onto the screen the same 2 fingers use tweezers to grab on to the protruding plastic tab on the screen and gently rest it back into its internal bracket. Once in the bracket you can gently pull up on the bracket until the screen bracket latch clicks into place.
7. Attach lens and test.
This is all there is to it. What you don't want to do with this focusing screen is rub it with anything that may polish off (or scratch) the exterior frosted surface, it is delicate. Water is not abrasive but cloths, Qtips, and even brushes are. If you did hurt this screen its not the end of the world, $40 can get you a new one on eBay.
Let me know if anyone has tried this method.
