I soak them using Woolite or Orvis or All Free and Clear. Use cold water, mix the detergent in, then add your block. Swoosh it around a bit and squeeze gently to get the piece entirely saturated. Then leave for a couple of hours, periodically squishing and squeezing gently. Then rinse well. Rinse again. Lay out on a towel and roll up to remove excess moisture. Dry flat. Repeat if you don't get all the stains out.
I prefer to use cold water since warm water can actually set in more stains than it removes.
If the stains don't come out by then, I would consider using Oxiclean. That stuff works nice, I do use it, but can be quite harsh on some fabrics.