When you draft a health care proxy, the goal is to pick an agent who will then act on your behalf. If you cannot make medical decisions, they make them for you.
For instance, if you have a stroke and you cannot speak to the doctors, they can decide what type of treatment you want. As you can imagine, it is very important to pick someone you trust explicitly. They could literally hold your life in their hands in some cases.
One important thing to remember is that you want to sit down and have a conversation with this person in advance. Make sure that they really want the job. It comes with a lot of responsibility and stress. They should never be surprised to find out that you chose them. In that sense, it's not the same as writing a will, when you can leave assets to people without telling them in advance. You want to work with the prospective agent to get this paperwork in place.
At the same time, the two of you may want to talk about what you want and what you expect. Tell them if you have any wishes that they may not anticipate. Maybe you don't want to be kept alive on life-support, for instance, or you do not want to be resuscitated if you pass away. Talk to the agent so that they understand your wishes, and so they can make the best possible choices when called upon.
Once you do this, then it is time to look into the legal steps you need to take to set everything up.