Get yourself a housing.
Buy everything.
Assemble.
Off the top of my head, you will also need:
LEDs, Cree XP-Gs, I'm assuming
Optics, or to give them their original name, Columnators.
Alternatively, you could try reflectors.
A constant current driver to run the LEDs.
A switch.
A bar / helmet mount.
A battery and charger combo.
You will also need, solder, and a soldering iron
thermal adhesive epoxy / paste.
Cable / wire / heatshrink and other sundries
power connectors
Can be quite satisfying to make your own light.
Good luck, and post what you make, if you do, be interested to see what you come up with.
🙂