Thanks for the note / acknowledgement.
Another easy question I see....
and one I have overcome a number of times.
There are a number of ways of going about this and to a large degree it depends on how you raise / support the car. As I am always working on the driveway, with a rubbish combination of trolley jack and axle stands the when the car is in the air all you have to do it try and grab the roadwheel, (I tend to leave mine on when working on the car) and that provides sufficient breaking force so that when you are turning the 6mm allen bolt, it will turn. Somehow I tend to have one hand on the ratchet, one on the extension and a shoulder pressed up against the tyre - works fine every time.
If, as I guess may well be the case with you Eddie, I would do as you suggest, put the car in gear, put the brakes on, I tend to use my handy spirit level between the brake pedal and the seat - just the right length - and press the brakes on - assuming the hub and calliper is in place?
I have seen other ways with a strong, flat bar with 2 holes drilled in - resting onto the ground, but with 2 bolts through into 2 of the wheel bolt holes and braced against the ground.
When you put the 6mm allen key in the bolt head - go carefully. It is often quite oily around there and the bolts can be tight. Make sure you are square on and nothing is going to slip / round off. Don't forget there is a metal plate shared between each pair that sometimes drops on the floor. When undoing the just apply constant, increasing torque to the head of the bolt and they slowly undo. The nice thing about having the road wheel on - is that you can then easily rotate the driveshaft flange into the next easy position to undo one or two of the bolts at the same time.