This video shows how I approach simple paper repairs - filling in missing areas and also dealing with small paper tears. Although I am not a qualified conservator and there are other ways to do this, I find this approach to be a practical method for straightforward in-situ repairs. The hard bit is often finding the right kind of paper to fill-in lost areas!
UPDATE: Thin light boxes like the one shown in the second part of this video are now available online for less than £15. Just search for 'A4 Ultra-thin Portable LED Light Box'.