Hi chap, just done same job for same reason.
Hints and tips (but no promises

)
Place window into frame, and make sure it is seated as best as poss, evenly all round, before doing anything with the string. Push the rubber in as far as it will go all round the frame, though still not clipped over.
I used wire instead of string.
Pull wire towards centre of window at all times, adjusting as you move.
Don't put too much pressure on window from outside, it hardly took any if seated right.
When the rubber starts to fold over the metal, only do about 6 inches, before seating it firmly from the inside. You have to follow it round, pulling in and seating down on the metal, especially round the corners.
I didnt use lubricant, but can't see major problem except making it easier to unseat????
Hope this helps, but if difficult to understand, pm me your number and i'll try to explain better.
Best of luck
Myk.