The installation is really a no-brainer and absolutely critical. The tweezers is necessary. The work environment should be as dust-free as possible - otherwise annoys you about lint in the viewfinder.
I have the installation carried out in the bathroom - with a respectful distance from the cat. In 1 minute the thing was in there.
Actually, everything was here also already said. But I want to mention again that the focusing screen does not only fit in the 40d but also in my 50d.