I use one bar of Ivory bath soap (until I have enough soap scraps to not need a new bar). I grate it with the fine side of a box grater and do a quick whiz with the food processor too. I haven't had any problems with it not dissolving in the washer, and I only use cold water.
The only downside I've run into is I'm as allergic to the Ivory scent as I am to most other scents. So I hold my breath when I only the container my homemade detergent is in and don't breathe again until after the soap is in the washer.
Jill