Sure, William.
These things are exceedingly easy to do. Much simpler than I anticipated.
First, find the artwork you want to copy. I'm going the Manesse Codex:
http://commons.wikimedia.org/wiki/Codex_Manesse
Use Microsoft Paint or something simple to crop the picture to what you want to copy.
Fold the paper print-out in half, half again, and half again. When you unfold it, it will now be divided into 8 equal zones.
Make your banner. Really you can use any cloth, but I recommend something thick so it lays still for you. You want to use a heavy linen, or a cotton broadcloth. Stay way from thin flimsy fabric as it will stretch and deform on you. We have found it is easier to go ahead and sew on the borders and the hanging-rod loops first, rather than paint and then do it. A single layer of fabric is all that is required.
We are using the cheap-o acrylic paints you can buy at Walmart, Michael's, or any craft store. They make a fabric medium you should mix with your paint that makes them more durable on fabric. But we have painted one without using it and it was fine. Now they won't take abuse (like rolling them up and unrolling them to take them to events and home). We are hanging ours on the wall and leaving them there. But if you are planning on toting them around a lot you will want to make sure to either use fabric paint or add the additive to make them suitable for fabrics. We found the fabric paints to be much more expensive than the regular bottles of acrylic paint, plus your colors were more limited. I think you're better off adding the fabric additive.
Using a yardstick, mark the paintable area of your banner into 8 zones just as you did with folding your paper. Now you have zones on your banner that match the zones on your paper.
Using the grid of the zones as a guide, use a chalk pencil (you can get them at any sewing store in a variety of colors) to draw out your picture. I draw the whole thing, including internal detail, even though you are going to paint over it all right away. I just need to see that the whole thing looks right in chalk before I get started with paint.
After you have drawn it out, paint the whole inside your item white. This gives you a good base for painting everything else.
After this dries, re-draw your picture over the white that you painted. Now you can start painting with colors. Do lighter colors first, as darker colors will cover any lighter-colored screw-ups better than vice-versa.
Feel free to go wild with the colors - they did in period. We are having an eye to trying to make things look like they should (wood is brown, leather is tan, etc.), but we are not worrying about whether the fabric color "matches" or anything like that. We are going for "color"!
Sharpie pens are useful for outlining things in black, or drawing black detail lines.
That's about it. It's amazing how quickly you can turn these things out.
Steve