There are two different types of aftermarket seats available.

Recliners (or adjustable), and the fix seats (commonly called racing seats).

In order to install a recliner seat, you will need

If is is a Recaro, the easiest install, there is a recaro adapter that will fit between the seat and the factory OEM mini mount/slider. (recaro part numbers, driver 68.83.19, passenger 68.83.29)

If you are installing another brand of recliner (Sparco, Cobra, Corbeau, Momo...) you will need a custom bracket that will replace your factory mount/slider and a slider (the slider need to match your seat)
The most popular of these custom brackets is manufactured by Wedge in California, there are also a few other custom brackets made by Sparco and Speedware,
All custom brackets come with a tab for your seat belt receptacle.

Picture of the wedge bracket


Now to install a racing fixed seat,

You will need a custom bracket, a slider and most likely a set of side mounts.

Regarding the airbag sensor, the best way to take care of it is to measure the resistance at the plug, and purchase a resistor of the same value.
This resistor will make the computer believe that the airbag is still connected.

For the passenger seat sensor, the one that disables the passenger airbag, the best solution is to remove the complete sensor pad and wiring from the factory seat and re install it in the new seat.

Hope this will help,

I am sure I forgot some stuff, so let me know and we will add to it.

Victor