As far as I know (my very limited Hungarian knowledge), hogy has also the meaning of what, for example hogy is hivják = what`s his name/ hogy vagy = what are you doing/how are you getting on? /how are you?
Both hogy and hogyan usually mean How, and in most cases both are possible, though hogy is more common.
e.g.: Hogy csináltad? = How did you do it?
Hogy/Hogyan ismerkedtetek meg? = How did you first meet?
Both words usually require some adverb as an answer, not just hogyan.
I may be wrong, but I would suggest you use hogy in all situations. It sounds more natural, and you can't make a mistake.
There are certain expressions where you can't use hogyan, only hogy:
Hogy vagy? = How are you?
Hogy hívják a kutyádat? = What's your dog called?
The question word Milyen suggests an adjective in the reply.