How do I backup my iphone onto my PC?

Download iTunes on your Windows machine, and then follow the steps here to back your iPhone up: How to back up your iPhone, iPad, and iPod touch - Apple Support

Via iTunes.

Yep, connect your iPhone to your computer using a Lightning to USB cable.

I download itunes and plugged in my iphone to the PC, but the desktop button to select backup iphone is greyed out. How is that resolved?