I just replaced a neck gasket and two wrist gaskets on a dry top. Doing it your self isn't that hard, and while the video posted earlier has some good advice, I would recommend using the method described here: NRS - Gasket Repair Instructions
You can glue to the old gasket, or it is pretty easy to remove the old gasket with an iron as shown in the video and then glue to the fabric directly. If you are going to remove the old gasket, it helps to have someone else with some cardboard to place between the fabric and the iron so that you don't melt any holes in the fabric. Lastly, expect to have the iron on the gasket for ~ 30-45 seconds before it is hot enough to remove easily.