How to follow up with potential PhD advisor met at conference?
Simply ask. Of course, your email should take in consideration PhD application instructions at his institution. If you can be admitted without an advisor, it changes the context. 

"Dear Dr. X,

It was a pleasure to meet you at conference Y and to discuss about Z. 

Since we met, I have read about your research area and [Insert question or comment].

Would it be possible to meet/skype to discuss the possibility of pursuing a PhD in your lab?"