Adding a staff

I have written a song with alto and soprano parts, and thus have 2 staffs-one for each part. I'd like to add an additional soprano part on its own staff. How do I create this additional staff to the other 2, so that there are 3 separate staffs? I'm sure there is a way, but I haven't been able to find anything to tell me how to do it (of course, there is probably a solution right in front of me but I don't see it!) Thanks!