If you've never done it before, and aren't too familiar with computers, patience is key. Make sure to remember/document exactly where everything is plugged in, etc. But the wiring and fitting isn't that complex. Just make sure the back panel stuff lines up correctly. Also ground yourself, a ground strap is preferred, but frequently touching a bare metal spot on the PC case will suffice.
BIOS settings won't carry over, so you may have to change a few things in there to get the thing to boot.

A friend bought a motherboard on eBay. Was listed as the same model and even looked the same but someone must of played around with the BIOS [hacked it?] as it still detected new mobo based hardware. That and a low ticking sound. Doesn't mean you got a dud, just he sort of got one.

From skrubol's comments, if you got a camera, take pictures as well.

Suggest you do a factory reset on the BIOS [see the mobo manual] just to make sure nothing is left over from the previous user - and of course upgrade the BIOS - if not already.

You can also put on [temporarily] stickers for the cables.

Good luck!