Thank you for your response!
I have used a regular hammer, a rivet tool from Tandy Leather that is sold with some of their rivets (I didn't care for the results of using this, but it could've been that I didn't hold it steady enough or did something wrong), and a mallet also from Tandy Leather. So far, the hammer and mallet have been best, but the rivets still don't look smooth on top.