Overview
Okonomiyaki (お好み焼き) is a popular pan-fried dish made from batter and cabbage. You can add a wide variety of toppings and ingredients, from meat and seafood to wasabi and cheese. The name reflects this flexibility: 'okonomi' means 'to one's liking.' Found across Japan, it is especially beloved in Hiroshima and Osaka.
Okonomiyaki is sometimes called an 'as-you-like-it pancake' in English. However, that can be misleading. While it uses batter cooked on a griddle, it is neither sweet nor fluffy. Instead, it is often filled with octopus, shrimp, pork, yam, or kimchi. A better comparison is to pizza.
In Japan, people usually eat okonomiyaki at specialty restaurants. Some have tables with built-in iron griddles (teppan), and customers cook their own meal from provided ingredients. Since this can be intimidating, here are the steps:

The batter and ingredients arrive in a bowl. You can see everything you ordered is fresh and good quality.

Mix the batter and ingredients thoroughly so everything is evenly distributed.

Turn on the gas, oil the cooking surface, and pour the mixture onto the hot griddle. Use short metal spatulas to shape it into a circle.

Once one side is cooked enough, use a spatula in each hand to flip it. This is the trickiest part—make sure it's firm enough to hold together.

When both sides are cooked, add toppings. First, brush on okonomiyaki sauce, which resembles Worcestershire sauce.

Add mayonnaise from a tall squeeze bottle in zigzag lines. Mayonnaise is a newer addition, so some shops may not have it.

Place shavings of smoked bonito (katsuobushi) on top using wooden pincers. The heat makes the shavings move and curl.

Sprinkle small flakes of aonori (dried seaweed) over the okonomiyaki with a little spoon. It's usually stored next to the katsuobushi.

The final step is the best: eat! Use the spatulas to break off pieces. Since the okonomiyaki stays on the griddle, every bite is fresh off the grill.