There are literally thousands of free printable vintage embroidery patterns out there - there's a whole group on flickr devoted to them. However a majority seem to be intended for linens or display, so - while I'm not against embroidery for embroidery's sake - I've tried to pick out a few designs which would work on clothing. I've also tried to choose simple designs, suitable for the beginner (like me).
The first two (both from love to sew) are super easy - the 'fluffy' outline means you don't even have to worry about your stitches being even. They're also ideal for embroidering on knits, as the effective 'zig zag' will stretch with the fabric. I can totally see them on a 50s cardigan.
This swallow is also constructed mainly of single, straight stitches. Without the words and clouds I think it would look great on a collar (print it reversed for the other side).
If you fancy trying backstitching an outline, you can hardly go wrong with this cute kitten! Perfect for a skirt pocket?
Transferring your printed pattern
Once you've printed your pattern there are a number of ways to transfer the design to your fabric.
As I did with my monogram, you can trace the design onto your fabric with a disappearing ink pen. This is one of the easiest methods, but it only works on a light coloured fabric which is thin enough to be able to see the design through.
You can get a special pencil specifically designed to create your own iron-on transfers - you trace the printed design (don't forget to print it out in reverse), then iron it onto the fabric.
Tailor's carbon paper transfer sheets are a good way to transfer onto a dark fabric - simply place the transfer paper between your fabric and the printed design, and trace the lines with a stylus, ballpoint pen or sharp pencil.
For knits or felts in dark colours, which won't take any of the above methods, trace the design onto tissue paper. Tack it in place on your garment, and stitch through both layers. When the embroidery is completed, simply tear away the tissue.