Salt can be used with hand dyeing to get some cool effects. But be forewarned. This effect is not something I can give you an exact recipe for. You're going to have to experiment a bit. If that doesn't appeal to you, skip this lesson. :)
As always, I'm going to assume you're familiar with preparing fabric, the basic supplies, and one-color baggie dyeing. If it's been a while, take a minute to review these. For this lesson I highly recommend having some urea on hand, too. And, obviously, you'll need some salt. I use Kosher Salt, because I have it on hand anyways for cooking. The coarser the salt, the more pronounced the effect -- many people like a very coarse salt. You're going to have to experiment a little, and see what you like. You'll also need a space where you can lay out your wet dyed fabric -- I cover a surface with a thick plastic dropcloth.
To start, you'll need to make your chemical water as usual, making sure to add urea to the water. The urea keeps the fabric moist longer, and (in my opinion) really helps with the salt effect. Get dye on your fabric with any technique you want -- dipping, painting, spraying, whatever. For the first time, you might want to keep it simple, and just dip the fabric in a plastic bucket that has a small amount of dye.
Lay the wet fabric out on your surface. I've found the fabric should be pretty damp, but not dripping. If it's too wet, the salt just dissolves and doesn't do much. If it's too dry, the dye can't move around, and the salt doesn't do much.
Now, toss some salt onto the fabric. I'm pretty generous with the salt, but I don't coat it completely. (I use a little less salt than you'd see on a pretzel, if that helps.) Let the salted fabric sit undisturbed for several hours, then rinse and wash as usual. You should find that the salt attracted the water in the fabric, which pulled dye along with it, making a cool spotted effect. This will look different if you hang the fabric up somewhere instead of leaving it on your surface, since gravity will give direction to the spots and streaks.
If the effect wasn't enough for you, try a coarser salt or more of it. If that still doesn't help, try a damper or drier fabric. But most importantly, have fun!