It's pretty much a take the old one out, clean out the channels, and install the new one using something lime 3M weather seal adhesive. Not sure but I think the seal is supposed to go in one way, ie. there's a front and a back.
If your channels are rusted out I think the fierostore now sells replacements.
Also look to see if it's got the drain hole mod. Basically a couple of small holes drilled in the front corners of the channel area through the SMC to allow any water that gets in there to drain down inside the A pillars. If not, search the net or on here and I'm sure you'll find a description of it.
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Dave
www.ltlfrari.com