Steam clean the rug first. You want to dye clean fibers, not dirty ones.
Spread out the plastic tarp or drop cloth. Put the rug on this. The plastic is to protect the floor beneath from the dye. If possible, take the rug outside. You may still want the cloth to protect the concrete from staining.
Dissolve the dye in hot water, according to your dye's package directions. Make sure you have a dye that works on wool. Start with a bucket of warm dye.
Wearing gloves, soak the cloth rags in the dye. Blot the dye-soaked rags onto the Berber rug. Be sure to apply pressure to get the dye all the way through the fibers to the base of the rug. Work your way across the rug, soaking it as evenly as possible. If not, you may end up with a spotty dye job.
Set the dye by steaming the rug with the steam cleaner. Wait for the rug to dry.