I found it was easier to get it more secure and get access to it if you put the board on one side of the car, but used to have it in the middle for ages.
What I do is:
Hold the buckle bit on top of the board about 6 inches from the edge of it, over one of the roofbars.
With the strap dangling down towards you, loop it round the roof bar twice (inside the roof bar, not on the very edge of it past the bit where it slots into the roof!)
Then throw the strap UNDER the board, across to the other side. Wrap it round the roof bar on that side twice.
Now throw it OVER the board, then under again from the other side and back over (you may have to vary the number of times you do this depending on how long the straps are)
Get the strap on the roof bar AS CLOSE to the board on each side as possible, and make sure it's pulled as tight as you can get it before chucking it over and under each time.
When you see you're running out of strap, put the opposite side from the buckle through the buckle and pull very hard. A good tip is once it's through, go round the other side and pull it so that it's through the strap but you're pulling backwards on it (you can get it much tighter than if you just pull it through and keep pulling forwards from that angle).
Twist any bit of strap left through the straps already tied down so it doesn't flap about as it'll make a really annoying noise!
Now repeat this process for the other strap on the other bar! It probably sounds quite complicated, but once you've done it 2/3 times it's a doddle (unless it's gale force winds and the board keeps flying off the car before you can strap it down!!!)
Hope this makes sense. Good luck!