Multiple ways you could back up, starting with manually copying files somewhere or writing them to a cd/dvd.

Made it into a guide so programs like deja dup can add a splash page

General advice on good practice when backing-up.

Things like making sure there is enough space on the backup disk, securing backups against fire and theft, off-site backup storage, frequency of backing-up (link to other topic).

Generally, everything you need to do to make a good, reliable backup.