Boot from the USB flash drive by rebooting your PC while pressing the key that opens the boot-device selection menu.

This will vary from device to device, but typically Enter, Esc, F10 or F12 are a good starting point.

Select the option that boots the PC from the USB flash drive.

This will typically be called something along the lines of USB: Mass Storage.

Some devices, like the Microsoft Surface tablets, don’t use a menu and just directly boot from USB when p. ex. the Volume-Down button is pressed while turning the device on.