I'm not sure if you still want to do this after reading the previous comments, but incase you do:
It's not that hard to do.
All you would need is a copy of Windows XP (an iso image or the physical one) and a purchased product key
You can buy the product key for it and then download an iso image and put in the purchased product key.
You boot up the install from a flash drive with an iso image or with the disk and you install.
But before you do install Windows XP, back up anything you may need if you have anything you want to a flash drive or external hard drive, and make sure any drivers you have installed are compatible for Windows XP.
After that, you can load the XP setup and it should format the drive and continue installing.
Just know XP is no longer supported by Microsoft anymore and if you want to continue using it, install some antivirus software, a VPN, and do the little trick where you can get security updates from POSReady 2009 in Windows XP until April 2019.
Please note that any updates from POSReady may mess up XP because it is from a different operating system and things are a little different in security between the two, so make frequent System Restore points before installing any security updates that may be released
Learn more about the trick and start doing it:
http://betanews.com/2014/05/26/how-to-continue-getting-free-security-updates-for-windows-xp-until-2019/
It's basically tricking the registry to connect to the POSReady security update server and that way Windows Update will pick up any updates
So yeah, basically back anything up you may need, make sure your hardware and what not, like drivers is compatible with XP, install some antivirus software, and do the registry trick to get security for the next 2 years
But that's up to you if you even still want to switch to XP..