You, tkamiya, speak truly. And as far a flipping the elements, this is not so much a problem with 'normals', but with something like a '24', watch out! And as the 'avatar' proclaims: do not force anything.
Benjiboy: collimating the lenses is 'done' by the very structure of the housing. Simply put them back the same way (and same side and with all rings and spacers) that they came out and you have no problems. The metal ridges on the housing guarantee proper placement. They CANNOT be out of line if you do this.
Good you brought this up, Benjiboy, because the HARDEST thing to master is proper CLEANING of elements. If you use straight alcohol you will not remove the static electricity and the dust will cling tenaciously (even after the glass LOOKS clean by simply glancing at it but not if held up so a lit bulb is behind it). I use straight household ammonia but you can also dilute it about 1+3 if you wish. OR... a bit of tap water with a drop or two of dish liquid in a cup is good: you must put in enough dish liquid to be able to remove the static electricity, but not so much that a residue of soap remains on the glass after wiping it off. It's a bit like washing windows.
I take a round element and do this: with CLEAN fingers spread a few drops of this cleaner on each side of the glass. Spread the liquid evenly. Then with an EXTREMELY CLEAN, SOFT tissue do this: take the element and hold it in one hand (I am left handed so I hold the element in my right hand) with only your index finger and thumb holding the sides of the element. Then, in gentle circular motion, place the tissue in your other hand and slowly turn the element as tissue contacts each side of the element. Keep turning the tissue to a clean part of the tissue (in order to prevent re-wetting dry glass). Get ALL the glass cleaned, leaving no part untouched. This not only removes all dust but treats is so that any remaining can be easily blown off (make certain that there is NO saliva near your lips!).
The FINAL test is to do this: hold up the element to a light bulb and confirm that it is TRULY spotless. Then you must act quickly to put it back BEFORE dust settles on the glass. The FINAL, FINAL test is to hold the whole, completed lens, at max aperture, up to a lightbulb and be rewarded with a truly clean lens!
This is easy to do if you have LOTS of practice (as I have). Remember, the static MUST be removed or you will NOT get dust-free glass. - David Lyga
I don't. What do you suppose I do when faced with a lens that's pretty old and not worth enough to spend money on professional repair? Well, in those cases, I carefully take it apart, do what I can, and carefully put it together the way they came apart. It works well enough and for the purpose intended, it's fine. If I mess up, oh well...
Obviously, I don't do this for my high-dollar Nikkors or anything else that matters.