First remove the elasticated band which came with the torch, then zip tie the torch to the helmet.



On my helmet this could hardly have been easier all it took was a single zip tie.Due to the design of my wife's helmet it was a little more complicated but still far from difficult.



You'll just have to see what works with your helmet.



Whilst I was at it I thought I'd also fit a rear light, in each case this was incredibly easy requiring only a single zip tie, most rear lights I've seen would be suitable.