Dear laura, I would offer that safety should be your first consideration. I would do it in an environment that you can safely exit from, should the need arise. Have your cellphone charged and close by. I would notify someone you trust about what you are doing--someone that you can summon quickly. Have an extra key in your car, if you can. You can never predict how angry a person might become if they think they are losing control.
Use I statements as much as possible. "I can't live with alcoholism" (for example).
Make it as straight-forward as possible. Beating around the bush won't make it any easier--and probably make it worse.
Expect that he will be angry, and will probably make all sorts of criticisms and heap all sorts of blame on you. Don't take the bait--it only escalates things.
He may try the technique of making promises of change; possibly crying and begging to give another chance. Do not waver---BUT, MAKE SURE THAT YOU ARE INTENDING TO ENFORCE THE BOUNDRY BEFORE YOU ASK HIM TO LEAVE. Don't say you want to break up if you don't mean it and can't follow through.
This is never an easy thing to do. Best of luck to you.
We will be here for you.