How to reset an iphone to factory settings?

Okay so a while ago I ejected my iphone from iTunes on my computer and now I really need to reset my phone to its factory settings.
I can't do it on my phone because it won't turn on no matter what I do.
And adding on to that, because I ejected my phone from iTunes, my device won't show up on iTunes when I connect my iphone to my computer with a USB.

First do Back up of your data. Launch Settings and tap on General. Tap on Reset. Tap on Erase All Content and Settings, and confirm the reset by tapping on Erase iPhone. In case you need to have a good security app for your smart phone, do try Leo Privacy Guard to make your device totally.

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