See my How to get access to fix the optical view finder lens? for the details. Here are the pictures. Here you see my plastic spudger pointing at the ploppy lens. If you wonder what that piece of paper with numbers and holes is: I put the screws through a piece of paper labeled with each "step" and oriented like on the step picture. Safeguarded with tape. That way, I do not lose screws and won't mix up the different lenghts.

I used this guide to fix my 600D, and it also worked like a charm.

The front lens of my viewfinder (the one closest to the eyepiece) has popped out of its plastic holder.