Well, the original bleacher made his first foray into dyeing. FB Rob gave me a few pointers...
In a nutshell:
1) remove spine. I used a single edge razor blade. It took maybe five minutes.
2) lay pennant face down on some old towels. I put the towels on a scrap of plywood.
3) mix dye. Rit tells you how to make 3 gallons. I figured 2 tbsp. for a gallon of water.
4) spray the back side liberally. When soaked in, spray again...and again...and again...and again. I'll bet I sprayed it 20 times. I used two full spray bottles of the dye mixture.
If you check the front, it will be "speckled" at first and eventually take on a more uniform color.
5) re-attach spine. I had a friend do it. A professional seamstress may have sewed a straighter line.
I may have gotten a little carried away...the dye bled a little onto the white paint because the paint was cracked. Other than that, no issues. Had the paint not been cracked, it probably would have been a total success. As it is, I give my effort a "B." I would, and may, do it again on a faded green Polo Grounds pennant.