Steaming milk is the art of incorporating air and heat into milk to create microfoam. It's what turns a simple espresso into a cappuccino, a latte, or a flat white.
Here's the technique:
Start with cold milk. Cold milk is the secret to microfoam. The colder it is, the more time you have to stretch the milk before it's too hot.
Submerge the steam wand. The tip should be just below the surface of the milk. Not on the surface. Not deep under. Just below.
Open the valve fully. This is the beginner mistake. They open the valve a little bit. That makes a hissing sound. You want a paper-tearing sound. That's the sound of air being incorporated into the milk.
Stretch the milk. For the first few seconds, you're stretching the milk. You're adding air. You should see the milk volume increase. You should hear the paper-tearing sound.
Submerge the wand. After about 5 seconds, lower the steam wand deeper into the milk. The tearing sound should stop. Now you're just heating and spinning the milk.
Temperature. Stop steaming when the milk reaches 140-150 degrees. If you go over 160, the milk will scald and taste bad. If you go under 130, it won't have the right texture.
Purge. After you're done, purge the steam wand. This clears the milk residue. If you don't purge, the wand will crust over and you'll have to soak it overnight.