The key lay in the back plate of one of my head torches, which happened to have several hooks and loops that you could pass a zip tie through.
Step 1: Materials needed
2. One or more Zip ties
3. A head torch with the right kind of back plate*
*My petzl was not suitable, what worked was a head torch I picked up in the local supermarket. They no longer stock them but when I wanted to make a helmet torch for my wife I found numerous cheap lights online which use exactly the same moulding for the back plate. In the end I got one like that pictured above from ebay for under £2 including postage if you search "head torch watt" you should find something similar.
Step 2: Fixing the light 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.