Hello Greggerly,
Welcome to Apple Support Communities!
I understand that you want to know how to set up the iCloud backup because whenever you try to backup your phone you're prompted with a message that your iCloud Storage is full. I am happy to help you out here.
The reason you might be seeing the message that your iCloud Storage is full is because your next backup size might be larger than what space you do have left.
To check your next backup size do the following:
Settings > Tap on your name > iCloud.
Scroll down to Backups and click on it.
Click on your device.
Below is an article that will help you manage your iCloud backups.
Manage your iCloud storage
You've also asked for the article on how to backup to iCloud so here is that article: How to back up your iPhone, iPad, and iPod touch
Let me know if that worked for you.
Thank you for using Apple Support Communities.
Take care!