A shampoo bar begins with the surfactant system. Everything else in the formula has to support how that system cleans, foams, rinses, processes, and ages on the shelf.
Start with the Primary Cleanser
Sodium cocoyl isethionate is a common starting point because it gives creamy foam and a mild skin feel. Sodium lauryl sulfoacetate can increase flash foam and cleansing power, but it usually needs balancing if the target customer expects a softer after-feel. Sodium coco sulfate is economical and easy to source, though it can make a bar feel more aggressive if it becomes the whole story.
Build Around the Processing Method
Pressed bars need powders and binders that compact cleanly. Extruded bars need a mass that can move through the equipment without tearing, sticking, or overheating. Poured syndet bars need a melt phase that behaves predictably without separating.
Prototype in Small Batches
Track batch temperature, compression feel, cure time, bar weight loss, foam character, and comb-through after rinsing. Those notes will matter more than a tidy ingredient list.