I just removed my headliner all together when it started to sag. I was going to fix it so I remember some of the repair steps but I don't remember the whole thing.
You'll have to remove the headliner. Mine was glued to the top so I had to be careful not to bend the headliner board. It came out pretty easy using a screwdriver. Next, you'll have to scrape the old foam off the headiner board.
I really don't remember the rest of it but you'll have to soak the cleaned headliner board w/ adhesive and put on only 1/2 the cloth so you can line it up right, then finish the other side. To put it back on the targa you can use velcro strips or use more glue, I'd use the velcro.
It goes something like that. Sorry I couldn't be more help.
I did mine myself. I took it all apart, went down to home depot and got some foam (about 1/8") and some spray glue. I cut the foam to match the cardboard headliner part, glued it to the cardboard, then I glued to felt back on the foam. Looks like new and it feels good to. It took about 2 hours.