I just died some BEP and Goose feathers with black rit. Here are my steps which are likely overkill.
I use large glass mason jar on top of its threaded ring inside of a large pot filled part with water to act as a double boiler to prevent the dye water from boiling. I usually bring the water to a boil and then turn the oven to the lowest setting I can. If I'm doing multiple colors I use multiple mason jars in the same pot.
Koolaide Is usually one packet per 8oz and I usually double up on the lighter colors (blue,yellow,pink) or use half the amount of water.
Rit I totally just guess but I should start figuring out how much I use so its repeatable. i would guess its about the same as a packet of koolaide.
I always make sure to presoak whatever I'm dying for at least 15 minutes. Bucktails I bleach and soap to remove oils and color but feathers I do nothing to. I let feathers soak 5-10 minutes and buck tail much longer. The longer you wait the richer the colors. If you look at my two oranges and two reds in the picture those are with the same koolaide, different soak times.