socal,
are you trying to convert a 50A electric range plug? Or are you trying to come off your main panel with a 50A 220v circuit that will be for grow. You just need to buy a small sub panel box. Bring in the 220v through one of the holes. Use the proper clamp. There are 2 main buss bars inside the box. Each one gets a "leg" of the 220v power. The ground gets connected to the terminal block that is mounted to the chassis of the inside of the box. Then you can start putting breakers in at that point. Balance them between the 2 legs (left and right opposing breakers of the same amperage preferably). If you have a 220v breaker(s) in there, then balance them with another 220v breaker of the same size or the same in 110v (20A 220v = 2 20A 110v). Then wire your neutrals to the other terminal block on the chassis of the box. At that point wire out from the box into outlet boxes for your 220 and 110 circuits. Remember, 220v plugs have no "neutral", only 2 hot legs and a ground. Hope that this helps. If that seems too much to wrap your head around then take the safe route and get some help.
-TF