If the document is a PDF, it will generally open in Preview by default. But you can ensure that you open the document in Preview by right clicking on the file, selecting Open With, and then selecting Preview.
If you don't have any signatures captured, you'll see a box that says "Click Here to Begin". Naturally, you'll want to click there to begin.
As soon as you click, be prepared to use the trackpad to write your signature. You can just use your finger for this.
When you're finished, simply hit a key on the keyboard and it will record your signature.
To use your newly created signature, click on the Signature icon once again. Then click on your signature and it will insert your signature onto the page. Once on the page you can place it where you'd like and resize it to fit.