Just finished installing a '91 Miata soft top. Truly aggravating. This was, however, my first time.
If you ever try it, you'll need:
-a LOT of patience.
-Expect it to take 2 full days. No distractions from the kids, etc.
-Go ahead and buy the new rain rail. You'll need it. I know, they want more for that dumb piece of plastic than you paid for the cheap top you bought off E-bay, but buy one anyway. Don't be a cheapa$$ like me.
-Buy the new cables as well. Yours are broken.
-Go ahead and remove the seats first thing.
-Get help for at least for the final install of the nuts holding the whole assembly to the rear deck.
-Don't get help for anything else. More than one person will surely loose bolts, tear something, etc.
-Don't remove anything by force. There are rivets, trim buttons, and screws hiding in very weird places. Everything comes off easily.
-Pay VERY close attention taking it apart, and VERY good track of all the screws, weatherstripping, etc. you pull off.
-Expect to make a trip to AutoZone near the end for plastic trim buttons (use new ones), rivets, and the screws you lost 'cause you didn't listen to me.
If you buy a cheap vinyl top (like me), they may send you 2 straps with grommets on the end and velcro tabs. The instructions for these are written by a Chinese first year English student who has never worked on a car. What the instructions are trying to tell you is that the straps go from the top rear "corner" of the roof to the rear deck under the vinyl, and define the crisp body lines at the rear corners. They also space #3 and #4 bows from each other. The velcro wraps around the bows, and the straps stick to the velcro. You could skip these, but the top will sag and look bad at the rear.
After 2 days of aggravation, you made it! Close the top and...Oh crap, it's 3" short from the latches. The vinyl needs stretching. I used a small ratchet strap wrapped around the steering wheel to stretch the top forward and latch it. Couple of days in the sun, and it should be fine.
BTW- I did use the cheapo rain rail that came with the cheapo top. It worked, but I loath the day that I will probably have to do this again.
Mine looks fabulous, but I am a VERY patient man, with a LOT of mechanical experience, and a willingness to do things like bodywork (which I know most of you hate).
The trim shop wanted $250 for the install. Probably should have paid them.
Good luck!