How to use shampoo soap bars

Shampoo soap bars can take a little while to get used to, they're different in format and nature to bottled shampoos.
Firstly they don't contain SLS. SLS or SLES stands for Sodium Lauryl Sulphate or Sodium Laureth Sulphate.
What is this?
SLES is an inexpensive and very effective foaming agent.^[1] SLES, sodium lauryl sulfate (SLS), ammonium lauryl sulfate (ALS), and sodium pareth sulfate are surfactants that are used in many cosmetic products for their cleaning and emulsifying properties. Wikipedia
So it is added to shampoos as a foaming agent. So the first thing you will notice is that shampoo soap bars don't foam in the same way as shampoo does. Psychologically this may make you feel like your hair isn't getting as clean as we've all been sold the line that foam is good.

So why don't they put it in?
Firstly it is a cheap ingredient often made from Palm oils. Secondly it is an irritant, although studies have shown that it is safe for human skin for the amount of time it is in contact for, there are an overwhelming amounts of anecdotal evidence from people whose dandruff and psoriasis has cleared up after stopping using products containing SLS.
Lush shampoo soap bars do contain SLS however their website defends their position on using SLS in their shampoo bars stating that it is safe due to the way you use a shampoo bar, foaming in your hands rather than applying the neat product direct to your scalp. Although even in this article defending SLS it admits that " it does have a potentially toxic effect on aquatic organisms"
Oh, that's ok then. 😒
As someone who continued to have a reaction to SLS in Lush shampoo bars, if this is an issue for you it would be best to avoid. However they note that their liquid shampoos do not contain it.

SLS is very good at stripping away all the oils and dirt away from your scalp giving you that squeaky clean feel. However the oils that you are labouriously stripping from your scalp and hair each day are actually very good for your hair and scalp. And so your body, which is trying to remain in balance, counteracts this by making more. Shampoo soap bars remove only the excess oils, however when you first switch it takes your body time to adjust to the new level of oil output needed. This is what is known as the Transition Period. How your hair and scalp react to this depends entirely on each person. You might find your hair is greasier, drier, flakier. This isn't the shampoo bar not working. This is your body getting used to not having to be constantly fighting against the SLS. How long this lasts depends on each person too, it might be a few days or weeks. Stick with it though, it does get better!

So how do you actually use a shampoo bar?
As we've established most shampoo  bars don't contain the super foam making ingredient so getting foam off the shampoo bar does take more effort, I foam it up in my hands (note new bars often take a bit more work till you get them going). Then run it into my scalp, then foam it up in my hands again, rub it into another bit of my scalp and so on till I've covered my scalp. So yes, it does take more time and effort than shampoo.
Then rinse it out. 
I then use very watered down apple cider vinegar to rinse out the residue. Approximately 4-6 drops in a mug then filled with water from the shower. Pouring it over the areas that I rubbed the soap into and again scrubbing the areas with my fingers to get any soap residue out.
I find I now no longer need to use a conditioner but my hair isn't very long, so if you do make sure you are rubbing the conditioner onto the ends of your hair only and not into the scalp as it can help trap the residue.

If you live in a hard water area or are finding that you constantly have that sticky residue feeling, try adjusting the amount of vinegar to suit or a clarifying wash every month or two months using baking soda. People who have heard of the No Poo method will be familiar with this.