there is a guide on here that is a mirror of ship killer website.
Ive done it about 5 times now, and can do it in about 20 mins!
The hardest bit is getting the metal ring out of the 2 clips on either side, BMW use a special tool but its night impossible without it.
TIP: when it gets to that stage, put the roof down half way so you can grab at the fabric and use the roof to pull it out of the clips, much easier.