It helps if you are already used to getting on corresponding edges and finishing your turns with a parallel skid, so practice that first.
Before you try to hockey stop, try turning with a wedge/snowplay the way you are but finish your turn up the hill to stop and stead of making your wedge wider. Do you snowplow/wedge turn in a very narrow wedge and keep turning until you skis are point across or even up the hill. You will come to a stop. Gradually try to do it sharper and sharper. You should start to get some skid and if your skis are in a narrow wedge, they should eventually start to get parallel (get on corresponding edges) and skid together.
Now, if you really want to hockey stop....
Get your skis parallel and ski straight down the fall line.
Stand very tall (extend) and lean a tad forward.
Quickly push both heels to one side while sinking your butt down fast.
You should stop in a skid with both skis parallel across the hill.