told me to just make some out of polymer clay, but I've never worked with it before and have no idea where to start.
One good place to start on the general idea of how to make buttons
from polymer clay is this page... it covers how to make them with strong enough holes, add shanks, how to wash/dry, etc., as well as lessons and links to lots of examples of clay buttons:http://www.glassattic.com/polymer/buttons.htm
That page doesn't go much into the various looks
that a button might have though (they could be molded, stamped, antiqued, gilded, caned, mokume gane-d, or many-many other techniques within polymer clay). What makes a flattish bead into a "button" after all is just the fact that it has two or more holes or a shank, etc., so it can be attached to something else as a closure.
I want it to match one of the colors of yarn I used, so is it better to use white clay and then paint it? If I paint it, do I bake it before or after? [/size]
Well, you can paint it if you want (must use an acrylic
paint), but generally any color in polymer clay items is built in simply by using different colors of clay (or clay colors can be mixed to whatever color/shade desired), or by mixing something else into the clay to color it before shaping and baking.
(If you do paint on top of the clay, bake it first, then wipe down with alcohol to remove any oils, then paint on several coats, drying after each. You can also bake the final item again briefly at about 250 to "harden" the paint, if you want.)
Other things can be used instead of paint on top of baked clay too --like alcohol-based inks, soft colored pencils, etc.)
If you simply want to make plain buttons of one color, then you may want to look only at the Buttons page above plus this page on mixing colors
of clay to get whatever color you want:http://www.glassattic.com/polymer/color.htm
....(if an exact
color/shade is vitally important, be sure to check the effects of baking on a few simple samples before baking your final buttons! --or use Kato brand clay which hardly changes at all ... some clays will darken for example, some will even darken a lot, though there are ways to prevent that... and you may have to figure out just how to bake polymer clay correctly too in whatever oven you have)
Oh, and btw, don't use Sculpey
clay for making buttons unless they are very thick and rounded ...that brand will probably break if stressed. Instead use Premo, FimoClassic, or Kato...or possibly FimoSoft.
Here are some pages with more info on those things. If you want to have something different or less simple in look, just say what that is and I can give more info and/or direct you to other info on that particular topic:paints, etc
polymer clay http://www.glassattic.com/polymer/baking.htm