If your transmitter programming is the same as my 1997 965, this should work.
The instructions were in my Owner's Manual, which fortunately came with the car. Sorta surprising since the car was an eBay purchase. It was easier for me to see the LED on the dash by programming my 4 transmitters after dark.

From the book, word for word:

New transmatters (max. 4) must be programmed to function with your alarm system. To do this:

-> Make sure all doors, hood and trunk/tailgate are closed.

-> Turn the ignition on and off 5 times within 10 seconds. On the fifth try, leave the ignition on.

-> Press either button on the transmitter.

NOTE: The first transmitter must be programmed within 15 seconds, and the others within 10 second intervals. When the alarm system accepts the codes from each transmitter, the LED on the dash will glow steadily for several seconds.

-> Switch the ignition off and test the transmitter(s).

If you did not get an Owner's Manual, see if you can purchase one here:

www.volvotechinfo.com OR call 1-800-25-VOLVO. Both will get you to Ken Cook Publishing, the company that does the manuals, including Volvo Service Manuals. I have found them quite helpful.

Good Luck,

Bob

:>)