Hey, Patrick. Welcome!
A couple things about getting downwind as a beginner:
- If you're really "stuck" and can't get downwind you can hold the sail in front of you with your hands on the mast or the uphaul and let the sail "flag" in front of you. This is just like safety position, only you lean the sail towards the front of the board until it's pointed downwind. As the board rotates you scooch both your feet around to the back of the board until you're standing with a foot on either rail (kind of like a stand-up paddle board) and then "flag" the sail straight toward the nose of the board. You'll slowly drift directly downwind.
- You're probably just getting used to being "powered up" with the sail and your tendency might be to sheet out (or let go with your back hand) when it gets too strong. Sailing down wind will be easier when you've powered up, gotten the board going at a decent speed, and _then_ turn the board downwind (as others have described). There isn't an easy way to point the board downwind, completely stopped, then "sheet in" and sail downwind. You, more or less, have to start from a beam reach and follow the instructions Greg listed. As you start to travel downwind the sail won't feel like it's trying to rip out of your hands since you've already gotten the board going at a good speed.