The process varies a lot depending on the computer that you are using. Traditionally, you would open the BIOS menu and change the order of drives in the BOOT menu. Newer computers use UEFI instead of BIOS, which needs to be accessed in a special way if you're using Windows 8.
Reboot your computer and look for the BIOS or BOOT menu key when the manufacturer’s logo appears. Press the setup key to open the menu, and change your boot order. The setup key is typically F2, F10, F11, or Del. Make sure that the USB drive is listed as the primary boot device.
Then you’ll see a screen saying: ‘Press any key to boot from CD or DVD ….’
Once you’re in the Setup program, select your language, time and currency format, and input method, and click Next. Click the Install Now button. Enter your Windows key if prompted, and read and accept the software licence. In the next screen, select 'Custom: Install Windows only (advanced)'.
What you'll see in the next screen depends on whether you're installing on a blank disk, or whether your disk has had an operating system installed on it previously. If it's a blank disk, you'll be shown a drive with 'Unallocated Space', so just select it and click Next. Windows will then start the installation process.
If your disk has had an operating system on it previously, or if you have more than one hard disk on your PC, you’ll be shown a screen with several disks and partitions. The disks will be called 'Drive 0', 'Drive 1', and so on, and if a disk has previously had a Windows installation, it will be split into partitions with labels such as 'Recovery', 'System', 'MSR', and 'Primary'.
Before you install Windows 10, you'll need to delete all these partitions by selecting each one and clicking the Delete icon. This will erase all data on the partitions, so quadruple-check that (a) you're deleting partitions from the correct drive; and (b) that you're absolutely sure you've already backed up everything you need.
Bear in mind that this may also delete your manufacturer's recovery partitions, so you won't be able to go back to your previous operating system. Once you've finished deleting partitions, make sure the drive you want to use for your Windows 10 installation is selected, and press Next to install.