alright mate, its pretty simple to do, you need a tool like a flat head screw driver but ideally something plastic so i doesnt damage anything. i use a plastic trim tool but i guess a plastic bicycle tyre remover tool will do or something along those lines.

start off on the inside top corner of the glass and carefully start to push the rubber under the metal flange. keep working your way along untill you feel it can be lifted out. just be careful not to force it.

putting it back in is a bit harder, you need a length of thin rope or really strong string. put the rubber on the glass, thread the string around the inside of the rubber, starting in the middle at the bottom all the way round then slightly over lap the ends back in the middle. you need a hand full left over at each end of the string.

place the bottom of the glass back in. now while holding the outside with one hand, putting pressure on it, pull the the string into the car working your way around the rubber. taking care to make sure that its pulling the rubber back over the metal flange.

if you can get some spray rubber lube, it will help it slot in nicely.

goodluck fella, its easier than it sounds