Wet hair thoroughly
Part hair and rub wet bar of soap on the scalp down that whole stripe of scalp you've exposed. I find it easier to grab the soap with my whole hand to where the widest flat part of the soap is what I'm rubbing on my scalp. It snags dreads less frenquently(ouch!) than if you use the smaller end.
move over one lock's width and separate hair again, rub wet bar of soap on the scalp.
move over...etc... etc. until whole head is done.
Wet hair a bit again. Rub the bar all over the rest of your hair.
Now take really wet hands and rub with your fingers all in your scalp. Really go at it if you want to take care of any itchies you have.
If you don't get big lather the first time then repeat. You'll defiinitely have big lather the second time! Mostly only newbies who're switching over or those dreadies who have done sweaty dirty work have to wash more than once. It's the oils that have built up. Once you get fluffy lather then you know you're dealing with squeaky clean hair and not a dirty oily mess.
I love my bar soap. I get mine from Mosalu on Etsy.
Just one little note of warning: as your bar wears down, check your scalp for small chunks of soap. It breaks off sometimes. You end up finding it pretty fast on your own anyway, but I just thought I'd give you a heads up. Soap flakes off the bar sometimes as it gets smaller and softer. It gets stuck in dreads. It happens. Just wet that area of your head and get it out. Seriously, it's no biggie. I've only had it happen to me twice in the past six months.