Vintage Seamstress,
Go to my Youtube channel (follow the links Miriam has posted above) and find my video about using a seam guide on a vintage machine. Half the reason I made that video was because people kept posting messages saying they couldn't work out how to guide the material with one hand. It isn't difficult.
I have the opposite problem to you. If I'm using a treadle I forget to use my right hand. Sometimes it waves about in the air, other times I am looking for a non-existent handle.