You may need an invisible zipper foot, available at most fabric stores, or on the net at Clotilde or Nancy's Notions.

Here's a tutorial that might be helpful:

http://sewiknit.blogspot.com/2006/03/invisible-zipper-tutorial.html

Apparently the Featherweight 221 series is a low shank machine. Here's another link with a low shank invisible zipper foot for sale, and the YKK brand version of the Coats plastic invisible zipper foot seen in the tutorial above.

Some tutorials for sewing invisible zippers with a regular zipper foot:

http://www.sewfastseweasy.com/items/sewing_machine_feet

http://www.simplicitynewlook.com/index.php?main_page=page&id=92

This tutorial uses a regular presser foot:

http://pieandbeer.blogspot.com/2007/04/invisible-zipper.html

HTH,
CMC

I use an invisible zipper foot (when you buy one they come with adaptations for different machines - at least mine did) with my 222 Singer and it works fine. Budster

You can get the manual on this page; it's only 3 dollars...

Here is a link that might be useful: Singer Sewing Machine Instructions