First, plug your device into iTunes. Once it's there, click the “Back Up” button on the right, under “Manually Back Up and Restore.”
iTunes Backup Screen options.
This way you have a bit of extra insurance in case it all goes wrong. If you don't have iTunes set to automatically check, click the “Check Now” button on the right hand side. Whether it checks automatically or you kick it off, it should prompt you with the following message:
Upgrading C3PO to iOS 8.
If you click either “Download Only” or “Download and Update”, you'll kick off the process. First make sure you have a full backup of your device on your computer, just in case. Then click the “Update” button (next to the “Check Now” button up above):
What you see in iTunes when the update is available.
This way you can do everything locally, and the plain download of iOS 8 might go a bit faster than the OTA process, and the file will certainly be smaller. Plus you won't need to remove a bunch of apps or photos or music just to upgrade. Do verify you've gone through the rest of the prep work, though. It'll come in handy later.