It depends on the hemmer foot... Is it a rolled hem foot (a funny little curlicue) or a blind hem foot (a vertical crosspiece). In the rolled hem foot, you sort of feed the fabric into the curlicue so that it rolls up and then gets stitched down. In the blindhem foot or edgestitching foot, the vertical bit is used to keep the fabric edge very close to the needle.

I found Ahles' Fine Machine Sewing (http://www.amazon.com/Fine-Machine-Sewing-Revised-Embellishing/dp/1561585866) very helpful for learning what to do with feet. I know the Half-Price books near me has the first edition for cheap.

Here are some charming old directions:
http://www.sewitgoes.net/smuse/attachments/foothem/nhhem.html
http://www.singer-featherweight.com/accesspages/acc_inst_foothemmer.html