It's only a "pleasure" if you've been doing it by hand with a darning egg and needle. And I expect you are going to need a hoop of some sort to keep your fabric snug - just like with a regular embroidery machine.
I've got the Singer "Teacher's guide" pup. 1925. While it has several pages on how to darn, all it says is "the 127 cover plate is attached with a screw" but shows no pictures.
My newer book, Singer Instructions for Machine Embroidery and Lace Work, pup. 1941 is of little help for your problem- it says to disengage the feed dogs, then install the proper cover plate based on the particular style of machine.
BTW - if you really want to learn something neat, get that second book. It shows some amazing work that can be done. I believe there is a site with a free download of it available.