What's the best way to prewash fabric? Cold or warm water? A full wash and dry, or just a soak? Do you use detergent?

If I were to prewash, I would wash the fabrics in the temperature of the water that I would be washing the quilt in and yes, I would use detergent. My opinion.

When I pre wash, I treat it just like I would clothes. Reds go in cold, the rest in warm, throw in Tide, no fabric softener tho for me in the dryer.

Same here!

I think it is the drying that shrinks, so I just soak.

I would wash everything the same way you'll wash/dry the finished quilt.

i wash the fabric how ever i think the finished product is going to be washed (if i know) always warm or hot. and then i throw it in a hot dryer, so any shrinkage is going to happen before i start sewing. flannels keep shrinking for numerous washes but i only pre-wash once. and if a fabric is bleeding i will wash it a second time but no more...if it still runs after the second wash it is used in a way it's not going to matter but i do not waste anymore time than 2 washes on it.

Sort the fabrics darks and lights same as clothes.
I usually use a partial load because I only wash a few yards at a time. I wash in warm water, rinse in cold. And use chemical free detergent and vinegar in my rinse water.

I wash mine in Woolite in pretty warm water, but I use the kitchen sink. I sort by similar color and just swish around a couple of minutes and then move to other sink to rinse and rinse until no more color shows in the water. Then I roll up in toweling to absorb excess water and then iron. I do the process this way to avoid getting so many thread strings and to keep collections together when I buy several pieces from one line of fabric. So far I've been very pleased with the results. If you buy large yardage pieces, this may not work for you. I think the largest piece I've washed this way was about 5 yards.

I use the kitchen sink and soak them. hand agitate and scrunching the fabric. then I wring it out and then hang dry in the house. either on a gate or on this wooden cloths hamper frame that I have and I will, if the kids are up and it is a long piece get my dd's little rocking chair and drape it on that too. i wash in barely warm water and i use soap in one of the soakings when i am messing the most with the fabric. I think that this helps the bleeders get the most bleeding out. and I can see when it "stops" bleeding.