If the 'Hangee' you're contacting is a Hangouts contact already, you can pull them up by searching their name or the Google+ circle they're in. If not, you must invite them (or vice versa) to Hangouts.
If the hangout is already started and they send you a hangouts message, you can reply thru the hangouts notification card on the watch. Sorry, I know that may not be what your asking.
If you are trying to start a brand new hangouts message like you would a SMS, I don't know of any way to do that yet .
Hi, well if I understand your question you want to send a Hangouts conversation .
To start a Hangouts conversation , the person you want to start it with has to have Hangouts .
If the contact has Hangouts then it appears in your Hangouts contacts with a Hangouts symbol next to it , you touch the contact and walla you have started a Hangouts conversation ...