What is the procedure to change a rear signal bulb on a 1991 LeSabre?

I assume you want to replace a directional bulb or stop light in the rear lens assembly.
Open trunk. Pull back interior lining. There are six wing nuts holding the lens assembly on. Remove these wing nuts, lens assembly will pops off giving you access to the bulbs.

How about 1997 model?

I just checked my '97 for you. Open the trunk, under the trunk lid behind the lights you remove four 10mm nuts and then the light assembly can be pulled out enough to twist out the old bulb and put in a new one.
Highland

Thanks Highlandlake.