My dad and I installed a top on my 91 Miata a few months ago. It is definitely not the easiest job ever, but it took us about 8 hours, and we'd never done one before. Honestly if you can find anyone to help you out, even if they've never done it, you can probably figure it out yourself and save the money. They come with directions, and there are some write-ups on the Miata sites about how to do it.
I would recommend getting one with the rain rail already installed though, makes life a lot easier.