Such a daft question, I'm almost embarrassed to ask!
I got on my bike today, for the first time in years, and I can't remember how to change gears. 
I remember that you start on a low number, go up the numbers to go faster, and down the numbers as you go uphill.
But the left handlebar has gears 1-2-3, and the right handlebar has gears 1-2-3-4-5-6.
How do I combine left and right?