This isn't a very hard thing to do. At all. A watermark is your name or a little symbol on the picture that 1) keeps people from stealing your photo and 2) is a "digital" way to sign your photograph. I use an app called Phonto in the app store to do my watermark.
When I watermark, I try to make it as little as I can without ruining the picture. I've done some bad watermarking on some of my older picures... Like making it too big, in front of something, or in the center of attention.
My current watermark:
If you're going to use phonto, you can use any of their fonts, or you can download them from online. It's kind of a long process, but you'll figure out how to do it. (If you can't figure out how to download from the Internet, leave a comment and I'll do a follow-up post.) Type what you want your watermark to be, and then shrink or grow it as you want. Pick a font, color, and put it on the picture. There you have it! A custom watermark that shows just how original you are(:




