Owain, Set up your straps as you have drawn in your photo to begin, You may want to move them later maybe not.
Use the washers provided to secure the bolts through the box into the tuttle head. Place the bolt in the T piece and gently spin with your fingers ensuring the Flange is flat on the bottom of the board. I have used this technique hundreds of times without cross threading.
Hope you have as much fun learning as I did. Two years on and still looking forward to my next session.