no, just put the string in the lip in the rubber giving a little overlap at the bottom, then hold the lot up to the gap and slowly pull the string. You will find that the rubber pulls itself over the metal lip and basically just carefully pull the string till it's all in.

You can at times give the glass a little slap with the palm of your hand to help ease it in if need.

Piece of piss mate, should take about 1 min!

Edit: If it's a fairly big piece of glass then make a loop in the string on all 4 edges of the glass, enough for you to pull on, then pull on the straight edge of the glass before trying to pull the corners in