Going on memory here since I generally just eyeball it: 50/50 Toasted sesame oil + green-top Soy Sauce
It's really important to buy real soy sauce, not just the crap substitute. Look at the ingredients to tell the difference. Real brewed soy sauce is water, soybeans, and salt. The fake garbage stuff will say "hydrolized soy proteins", alcohol, etc.
Once you get that base, the seasonings can be anything. I use a lot of garlic powder, ground white pepper, Chinese 5-spice powder, and some kind of heat. Chili oil works great for adding heat without additional flavors. Sriracha for a bit of Vietnamese twist or Gojuchang for some Korean influence. Or just some red pepper flakes if you're not that interested in heat. Adding a splash or two of rice wine vinegar will brighten it up a bit, as will some Mirin Japanese cooking "wine". Put it all in a jar and shake the crap out of it to mix.