I helped a buddy replace a top on a 3 Series BMW and it was not an easy job. We're both anal as hell about the details, so we video taped the de-install in hopes of using the info on the install. While it did help, it was still very difficult. Once we had it off, the prep to the frame is key...sanding all the glue off, and then priming and painting the frame before the install. He used a genuine BMW top and it was a perfect fit...be prepared to use a little steam and stretching to get it right around the window areas, as that was the toughest part.
We set up a laser line on the ceiling of his garage to line up with pre-measured marks we put on the new top to center it. There was a good amount of gluing and hidden wires that make the top taunt so it showed no wrinkles.
All in all, it was a pretty cool thing to do and the job came out as good as a shop would do...which is why he did it. He saw 'professional' jobs done that looked horrible with wrinkles, mis-aligned tops, and glue showing...not to mention leaks around the windows during the rain.
It can be done, just make sure you have a garage to do it in so there is no need to dodge rain drops if the weather changes during the install. It took us 3 days to complete...that included drying time for the paint and PLENTY of beer breaks to take the edge off the occasional frustration.
If it was my car, I'd pay to have it done!