I switched from traditional liquid shampoo to my home made shampoo bars years ago. I love my shampoo bars but I know it might feel strange switching from your classic plastic bottle to a bar.
I recommend using an apple cider vinegar rinse before you switch to a shampoo bar. That way any build up from your old shampoo goes away. Traditional shampoos can leave a residue on your hair so to truly test the bar I think a rinse helps. Just use a couple tablespoons to a quarter cup of ACV (or white vinegar) and poor it on your head straight or mixed with water, then rinse it off. I do this about once a month now just because it leaves your hair super soft. I often add essential oils to the vinegar for a nice smell. Rinsing with vinegar also helps remove any build up from products such as hairspray.
To use a shampoo bar all you have to do is get it wet and lather it up on your hands. Once your hands are all sudsy rub your hands through your hair like a normal liquid shampoo. Then rinse it off really well. Any unrinsed shampoo will leave your hair feeling greasy. It's that easy. The only thing not to do it rub the bar straight on your head, it won't feel great and it won't work well.
If a shampoo bar isn't working for you, it could be your water. When I moved my shampoo bar stopped feeling great and my hair was a mess. My new well water doesn't have a filter or softener so it took some time for my hair to adjust to the new water and now my shampoo bar is back to being perfection.