Gardeners frequently use liquid manure (they call it “manure tea”) and every gardener does it differently. The most common process is to take a shovel full of composted manure (raw or fresh manure should not be used on edible crops) and place it into a container. Fill the container with water and let it sit. Some gardeners pour off the brown liquid (the “manure tea”) after a few days, others let it sit for a month or more.
Diluting the manure tea is accomplished by adding water until the mix is light brown in color. Until you become experienced at it, I recommend that you take the diluted mixture and apply to only one plant. Wait for a couple of days and check the plant for signs of scorch or burning. If the plant does show those signs, you need to dilute the manure tea even more. If the plant is fine, then it is safe to apply around your other plants.
Some gardeners use foliar application–that is spraying the manure tea directly onto the plant itself. This is not recommended for edible crops because manure can contain pathogens and cause illness if the pathogens are still on the crop when eaten. Using composted manure will greatly reduce this risk, but not eliminate it entirely. A good article to read about pathogens and using manure on food crops is:
Manure and Compost Utilization on Fruit and Vegetable Crops
Jill Heemstra, University of Nebraska