I haven't tried it, but I think that you just use the same amount of salt as you do oils. So say you use a total of 3.5lbs oil, then at trace you would add 3.5lbs of salt. And I think that you have to cut it when it is warm or it will get super hard.
I think that I have a "recipe" somewhere. .. let me see if I can find it.
I make them all the time and just love them. They are also very good sellers for me. The secret is to use a large % of coconut oil in your recipe as coconut oil does not react to the salt by diminishing the lather.
The lather in a salt bar is very lotion-y and does not dry your skin at all.
I use 90% CO, 5% babassu and 5% shea and equal amounts of salts to my oils, i.e. if I have 40 oz of oils, I put in 40 oz. of salt. You can use any salts except epsom salt . Some say you can not use dead sea salt while others will use it. My experience shows that it greatly diminishes the lathering properties and makes the soap feel kind of rubbery. I don't like it but will dissolve a bit in my water for my lye solution for some label appeal. I add my salts at trace but have read where some dilute the salts in the water they add their lye to.