Heat a small pot over moderate heat and add the soy sauce, water, ginger, garlic, garlic salt, sugar, and fish sauce.
Bring to a simmer.
Once simmering, mix the cornstarch into the 1/4 cup of cold water; mix until well blended and there are no longer any white chunks.
While whisking the teriyaki sauce, slowly add the cornstarch mix to thicken the sauce.
Allow to simmer for a minute, then remove from heat.
Store in a heat-safe container. Homemade teriyaki sauce can last for weeks and weeks, so you'll likely finish it before its quality diminishes.