How to backup files and MySQL database (this is what your entire SE site is made out of, for those who may not know)
Before installing any 3rd party plugins (because I expect it to mess up my styling in some way), I do a complete backup of my files and...  moreHow to backup files and MySQL database (this is what your entire SE site is made out of, for those who may not know)
Before installing any 3rd party plugins (because I expect it to mess up my styling in some way), I do a complete backup of my files and database.  When a plugin changed my site to something unusable, I just delete the entire site and restore my backups (which, to my delight, is just the files and the database.)
Backup Files

Go to Cpanel
Click on File Manager
Click on public_html folder
Go to Settings (upper right corner) and make sure Show Hidden Files (dotfiles) is checked.  If not, check it.
Click Select All (all files and folders in your public_html should be selected.)
Click Compress
Select Zip archive
Click Compress File(s)
...wait a while
Name your zip file (which will show up in the public_html folder) and download/move it to a backup location of your choice.

Backup database

Go to Cpanel
Click on phpMyAdmin
Click on the database associated with your SE...    less