Polythene covering

I have a Freecycle greenhouse, old and in need of care - but free..anyway, the glass panes are very fragile and a number have splintered and broken. I have got a nice big roll of polythene sheet and am thinking of throwing this over the greenhouse, or indeed, remove all the glass and just use the polythene sheet.
Do any of you have experience or thoughts on this? Just not sure how to secure the polythene at ground level.