I am not sure if you are talking about vinyl cove or wood cove so I will give you a pointer on both.
Vinyl cove: adhere the first piece tight into the corner. Use a utility knife to cut a 45degree angle on the bottom curved area. Shave the excess off the back and adhere to wall overlapping the first piece.
Wood cove: Always place cove mold onto your saw exactly the way it will sit on the wall when cutting. Take your time and think it through before cutting.