If you have a key, insert it and turn it in the lock to "ACC." Insert a paper clip into the tiny round hole and push against the spring loading. Pull on the key and the lock cylinder comes out. Reach inside the dash and push the switch body toward the dash against the spring loading, and turn it counterclockwise (apparently clockwise from the outside) about 15 degrees. The whole thing comes apart, bezel and switch body and an outer cylinder part too.
Since you don't have a key, I'd say either get a locksmith to make a new one, or bang a screwdriver in the slot and use it like a key. But before you do that, wait on this thread for the possibility that someone might have a better idea. Once you resort to destructive force, there might be no way back.
Edit: I just remembered reading a news story about the rash of thefts of '90's and '00's Hondas and Toyotas, that the kids take a random key and file it down to make a master key which works on multiple vehicles. My understanding is that they just smooth off the high spots, they don't completely file them down to nothing. I don't know if this will work in an old car, but I don't see why not.
Remember, those keys with the same pattern on both edges, only one edge at a time is working. With the vintage Fords with the two-sided keys, you can take a key blank and cut your igntion key on one side and your trunk key on the other.