Well, I am REALLY new to this and have only done 3 discs. Here is what I did:
I had a white, star plastic Innova disc.
I used Rit, Sunshine Orange powder.
I went with 1 packet to four cups (same as NoThingness said, "2 cups H2O and 1 Tbsn powder" suggestion for this color). You might need to increase that mix depending on the size of the pan.
I boiled the water in a pan, added the dye, put in 1 teeny, tiny drop of dish soap (I heard that would help prevent spotting due to the undissolved powder and I guess it did).
I put some cut to size bamboo chopsticks in the bottom of the pan (you obviously don't want the plastic to touch the bottom of the pan because it would stick and melt).
I lowered the temperature to 120°F and dunked my first disc.
I put the disc in top down so that the water filled the disc like a bowl and kept it submerged.
I stirred it occaisionally and kept spinning the disc about every 2 minutes to keep the chopsticks from making marks and that also seemed to help keep the dye from settling in the bottom of the pan.
I let it soak for 20 minutes (at NoThingness' suggestion for that color) but occaisionally checked it for the right color and could have pulled it earlier.
I took out the disc over an old towel to keep it from dripping on everything and then rinsed it off in room temp water, then dried it with a towel.
Finally, I heard that tossing it in the Freezer for about 24 hours would help the overall longevity of the color, so I did that.
Suggestions: Use rubber gloves or some clever tongs to handle the stirring, flipping, spinning and removal of the disc, this stuff really stains. Make sure the disc is clean and dry by using dish soap and then thoroughly rinsing it in warm water until it is squeaky clean (no soap residue). SAVE THAT DYE - pour it out of the pan through a funnel into a jar or other lidded container as it's just as good in the future.
So you know, I tried the Rit Fuchsia powder the next day, using the same formula on another white Star plastic disc and got nothing but an ugly, pale rust color. It didn't make any sense at all. I tried soaking it longer, I tried adding more dye to the mix and it just wouldn't work. I re-dunked the same disc in the previous days Orange mix and it came out beautifully. There are obviously some unpredicatble factors and many variations in mixing for different colors... and some evidence that Rit may have recently changed their product