Trim Tex makes a bead for this very thing if your drywall is 1/2" you need a 1/2" tearaway bead it's very easy to use and you can get it at some big box stores or a local drywall supplier. you can go to trimtex .com to see it. It is great where 2 different surfaces meet. You mud just as you normally would but there is a lip on this bead that keeps your mud from getting on the other surface. When the mud is dry tear off the strip and you have a straight line.