How do you say 'eat now' in french?

If the question is referring to the command in the following context, then the command is "Mange(z)-le maintenant" with the "z" being added for the "vous" form and not being present for the "tu" form:

Person 1: I'm not that hungry. (Je n'ai pas tant de faim.)

Person 2: I don't care, eat it now! (Ça ne m'importe pas, mange-le maintenant.)

If you have a sentence with the Words "eat now" in sequence such as the example below, conjugate "manger" to the proper subject and use "maintenant" to mean "now":

Person 1: It's 12:00, so I eat now. (Il est midi, ensuite je mange maintenant.)