Sulfur was pretty much the only treatment for psoriasis for a few thousand years until the age of steroids. It's unclear to me why it has become less prevalent.

It's a natural keratolytic, meaning that it dissolves dead skin cells. It's also been shown (e.g. this paper, which also cites a bunch of related papers; or this paper, which looks at sulfur spas) to have anti-inflammatory properties in the skin, which is of course highly relevant to psoriasis.

There are some products (e.g. Jasön has a sulfur shampoo) that contain sulfur. But you can also buy it as a powder. I suppose you can just add it to your bath. I can't tell you what the dosage should be, however.

You can also add it to lotions. Sulfur is used to treat mange and other animal conditions, and so you can get sulfur lotions like this one meant for that purpose, which should be fine for humans if you dilute if with a normal moisturizer. NuStock is 73% sulfur, you want something closer to 10%.