- Download Windows 10 Technical Preview from here (you need to join the Insider program first) or directly from here.
- Connect your pen drive and Open the Boot Camp Assistant:
- Click continue, select how your drive will be split between OS X and Windows:
- Follow the instructions until it reboots to the Windows installer
- Follow the Windows installer instructions as well. If it says that Windows cannot be installed in the partition that Boot Camp created, delete it and create one from the Windows installer
- If you want to boot into Mac OS X again, use the Boot Camp manager in the system tray on Windows:
- To go to Windows from OS X, use the Startup Disk preference pane.
Enjoy! Here's my own Air running Dark Souls, a PC-only game: