How do you tell him you want a divorce

You just tell him and mean it.

Before you jump to filling in papers,its always better to talk out with each other and see if both of you can make it work.Try MC.Bringing it out on the table as to what is not right in the marriage or how you would like things to be different can make a lot of difference if both partners want to give it a try, else,divorce is an option.
Do all you can and if nothing works,you would know that you tried everything and move through divorce w/o any regrets.