Step1
Identify reasons/motivations: Why do you think your child should leave (ex. 1000yrs old and has never been on own, causing problems in family, not following through with rent, etc)? Write them down.
Step2
Identify reasons/motivations: Why does your child feel he/she should stay (ex not financially stable, has little social life, has a built in maid)? Write them down.
Step3
Identify if you are encouraging your child to stay by your actions. For example, if you are cooking and cleaning for them, it makes it more desirable to stay then go. If you are, then stop!! Make them start fending for themselves, as they will when on their own. Forwarn them "You will now have to do X on your own". And stick to it, no matter what. If they are on their own, they will have weeks of laundry piled up and have to either wash or buy more.
Step4
If you are not charging rent and they are done with school and employed, then begin to charge a reasonable amount. If they have to pay and share space with you, then they might as well pay and be on their own.
Step5
Sit down and talk about what your child thoughts are about being on own. Is it finances, complacency, fear.
Step6
Offer your assistance: Look in papers, go with to view housing, go shopping and pick up needed things. For exaple, if you are at a garage sale and can afford it, pick up a dish set or pans. You are not to fund the contents of their new place, but may help them to feel it is on less item on their list of "things to do"
Step7
If it is finaces or fear, seek info about possible roomate situations.
Step8
Regardless of reason, you will need to establish rules through communication (rent, chores, or statement of when you need them out by).
Offer support and assistance to help them get to point where they are seeking to move out on own.
(firmness and support)
Authority 
