Stretch the tights over an object such as a shoe, a plastic soda bottle or shampoo bottle so they are pulled tight, the way they would be if you were wearing them. Make sure its something that won't be ruined if you accidentally get fingernail polish on it. Also make sure its something with a smooth surface so you don't cause more snags,
Apply a generous coat of clear fingernail polish to the edges of the snag or hole. This will keep it from tearing further.
Let the area dry for approximately 30 minutes, then apply a second coat of clear fingernail polish.