Corn gluten meal is a high protein that makes it about 10 percent nitrogen. Apply it as you would a fertilizer, about 20 pounds per 1,000 square feet, to the top of the soil. You may scratch it in lightly and water enough for the seeds to sprout. Then hope it doesn't rain because the meal needs to stay dry. The seeds will sprout, but the roots will not develop.
If your timing is too early for the weed seeds and there was continuous moisture, such as rainy spell, the microbes will eat the meal before it can do its thing to the weed roots. Then you will have healthy weeds. The trick is knowing when the native weed seeds are about to sprout. Once a root is established, the meal has no effect. This is an excellent fertilizer for perennials such as strawberries, blackberries, and lawns.
You can read more about corn gluten as a herbicide by visiting the Iowa State University Extension and its Corn Gluten Meal Research Page