You heat some water and start adding salt. I've not bother to measure it in most cases as the ideal according the vast majority of sources is to add salt until an egg floats in the water. That would be an uncooked and unbroken egg.
Gently heat a quart or so of water and add a half cup of salt. You'll need more than that but it's a good place to start. Once the salt dissolves see if the egg floats. If it doesn't, add more salt until it does.
Once the egg floats you're brine is strong enough. Set it aside and let it cool while you crack the olives.