you can disregard the rest because this webpage is so far the best one i've seen on this subject: http://nomediakings.org/doityourself/how...
make a screen for each tone. print from light to dark, one on top of the other, this way the darker color can cover the lighter one, make your lighter color screen printable area a bit bigger then it should be, so that the darker color can overlap it when overprint.
your art work for the screen should have a marker of somekind for tiling purpose. see source for a rough idea.
Okay maybe the above does not help, how bout a step by step guide to silkscreen printing
and how to expose photo emulsion for your silikscreen (u need a dark room for this)
and try the How to build a printing unit for precise registration, since you waht to print in multiple tones.
"Clean up an image" is really subjective. You can spend hours working on an image in PS.
If a client gives us an image they found "online" that already brings out the red flags in regards to image quality and copyright. As for scanning your own images, the better quality scan you can achieve, the less work you'll need to do in PS to clean it up.
By it's nature, silk screen printing is usually down with very rough screens so the image doesn't need to be all that great in most instances. We have also silk screened a highly detailed Japanese logo onto wood using a very tight screen. For that job we started with the clients very high quality eps file.