First, you can sign up for the army in the community where your hukou is located or in the armed forces department.
Second, if you are a college student, you can go to the armed forces department where the school is located to sign up for the army.
Third, those who have been away from home for several years must have a certificate from the local police station, that is, they must have a temporary residence permit for more than three years, or they can sign up to be soldiers in other places.
I hope I can help you. If you don't understand, you can keep asking!