I played tonight with a hemmer foot. Figured it out after a bit and thankfully I have an old Singer manual that shows how it works. It was doing great until I hit the double seam in the pants which it was not happy about. So I've hopefully learned 3 things tonight: #1: Read the manual for guidance but don't expect it to work the first time out of the gate; #2: Find a wider hemmer foot for the next pair of pants and #3: Not all short shank hemmers will fit all machines. The last one is because the Necchi hemmer didn't fit the Bel Air but a Griest one did (now to find the Griest attachments - that'll be a job.)
Chris