I borrowed (and reprinted) the directions below directly from MacLife's website:

If you're ever in a similar boat, give the following tutorial a shot. After I saved each signed file, I was able to email them directly back to our mortgage broker in about 10 minutes! There was no waiting for my printer to slooooowly scan each page and even better was the fact that I didn't have to burn through a drop of any of my printer's (seriously overpriced) toner cartridges. Then I opened each one of the documents and added in all the signatures and dates, carefully resizing them as needed and placing them in the correct spot. I went ahead and created the necessary digital versions for my wife and myself and also a handwritten scan of the current date. Then I remembered a great feature using the Mac's Preview software where you can create a digital version of your signature by holding a piece of paper (containing a written version of your signature or whatever else) in front of your Mac's web cam.

The PDFs were also legal sized (and I did not have any legal paper at home) so I faced the prospect of printing out 6 letter sized sheets to print, sign and re-scan. The normal (tedious) procedure would be to print out the PDFs, sign and date them, re-scan them, and email them back to our broker. I'm currently in the process of refinancing my mortgage and received an email from our broker that three of the documents we had recently sent him were left unsigned and undated. I may have written a post about this feature before.but it saved me a bunch of time last night so I figured it would be well worth revisiting here.

How to insert signature in pdf preview