I recently took the plunge and blocked an IRR quilt. I had NO idea if all the fabrics had been washed or not since others had worked on it. It is basically reds, bricks, browns and creams. It really really needed to be blocked, so I decided to just go ahead, close my eyes and go for it. I ran some water in the bathtub (cold) and threw in 3-4 color catchers and then dunked the quilt. I totally soaked it then pulled the plug and drained the water. I then carefully picked it up out of the tub and put it in the washer. I spun it for just a few minutes to get the bulk of the water out.
I spread towels, then sheets on the carpeted floor. Placed the quilt on top and carefully pinned it every 1" or so- measuring it with a metal tape measure as I went to insure that it was square. I covered the top of it with another sheet when I was finished. It dried in about 24 hrs. It was perfectly square and flat when I unpinned it.
The next time I do this, I will do a couple of things differently.
1. I'll "dip" a questionable fabric in the water prior to dunking the entire quilt (although my fabs did not run- the color catchers WERE brown when I drained the water from the tub. This will give me an opportunity to make sure whether it will run or not.
2. I'll go buy some foam core (4 x 8) sheets to lay on the floor to pin the quilt. It will block much better on a hard surface than on carpet.
3. If you don't want to "dunk"....misting might be a way to go
Otherwise...it works perfectly and really makes the quilt lay/hang much better...good luck!