I will be writing mostly about Singers since that is what I know and have old booklets about. If your machine isn't a Singer that's ok--most of them follow the same general principles. I'll be demonstrating on my 15-91 pictured above and sometimes a 201. I have some diagrams of other machines I'll be using.
I'm not going to tell you how to rewire or clean your machine. Other places do that; such as ISMACS or the many Yahoo groups for vintage machines. You'll find varying information. Some people will say to clean with one thing and others use something else. Just try and stay away from household cleaners which can make the gold decals turn silver if not totally remove them. You'll notice my 15-91 isn't perfect. It has nicks in the paint. I tried to clean a thick layer of gunk off it and couldn't get some of it without ruining the decals so you can see how they're cleaned around but not on. I don't mind. It's a workhorse of a sewing machine not a museum model. If you can, identify your machine. If it's a Singer it will have a serial number on an oval plate on the right front side of the bed. You can go to the Singer site and identify it. Once you have the ID you can hopefully find a manual. ISMACS has some. Also Sew-Classic. For free. I'm a member of the Vintage Singer group on Yahoo where there is a section of files with manuals and loads of helpful information.