Dear SIL has a new Macbook Air, so I thought I’d post a few tips here.
To create a signature for use with documents, click on the Edit icon (the one with the pencil on it), or hit Shift + CMD + A.
Then click on the signature icon (see (1) in the screenshot below, and then click on ‘Create Signature with Built-in iSight.
Draw your signature on a piece of paper, and hold it up to the webcam (iSight). You will need to move it closer or further away to get it to focus the way you want, however with a bit of fiddling you will get a signature that will look acceptable on documents.