It's time to see if all the hard work paid off. If you didn't check over your work at the beginning of the last section, now is definitely the time.
Carefully move your PC case around so that you can clearly see the front panel again. At this point, take some time to clear to one side all your tools, loose parts and packaging, so that you have room to move the keyboard and mouse into the main area of the work surface.
Check the mains power cables to your monitor, PC Case (and speakers if necessary) are plugged in and ready and switch them on at the wall. The PC case should not power on at this point, if it does, simply press (and if necessary, hold for a few seconds) the POWER button on the front panel until it switches off.
Switch on the Monitor (check the manual for help with this if necessary.) The Monitor should show some indication of being 'on' by displaying a small green/amber light on the front panel.
If there are any loud or disturbing noises at this point disconnect the power at the wall socket immediately and refer to the Troubleshooting section.
Check that the CPU fan is spinning to prevent heat damage to the CPU. You should be able to hear the faint 'rushing' noise of some fans and the noise of the HDD spinning up.
The 'power' light on the PC case should illuminate and the HDD light may flicker a little.
After a few seconds, the Monitor will begin to display various texts and eventually stop at a message similar to - Unable to load Operating system - or similar.
This is an excellent response and demonstrates that the system is ready to have the Operating System installed.