Clean the plastics really good.
Get an appropriate sized pot, something that will hold enough liquid to get the plastics submerged.
Fill the pot with water and bring it to a slow boil.
Add a bottle of RIT dye to the water and stir it in.
Reduce heat to a simmer
Now add your super clean plastics.
Keep the plastics in the pot on simmer for as long as required. Note that the longer your soak the plastics the deeper the dye will penetrate.
Oh and you might want to do this outside maybe on the side burner of your BBQ.
I cooked my plastics for approx 1hr.
I should also mention that some plastics won't take the dye. I tried to dye max flow plastics and it didn't work. I think
@PA Dan tried to dye a bar scabbard and it didn't work either