I converted my 1991 940 to electric windows using parts from a 1990 940. In my case the fuse and power window relay were already in place and wired in. Check the fuse box behind the ashtray to see if the fuse and relay are there. If they are then all of the wiring under the dash will be there which saves a lot of work. You will need to obtain the wiring looms for both sides of the car that have the additional wiring for power windows. These plug into a large rectangle plug under the dash on each side. You remove the existing wiring looms from your doors and unplug them from those large plugs and then plug the replacement loom. Bitch of a spot to get to but it's doable. There is one loom for each side running to both doors (ie. one loom for the right side and one for the left).
Obviously you'll need the power window motors and mechanisms for the doors and the switches.
It's a pretty straightforward job as long as the car is prewired because everything is pretty much plug and play. If not then you've got a significant amount of extra wiring work to do.