Never done that before, any things to keep in mind
Steps i would do:
Plug device to pc
Do to device
Go to backup location
Format storage ( how??)
Yes, correct. Copy your data to PC.
Then do it using advanced wipe in TWRP recovery: Wipe\Advanced Wipe - be shure to check correct partition to wipe, wipe, reboot.
Of course, you had to have TWRP recovery installed. And don't forget to make full backup in recovery.
Also you can search for big hidden files using file explorer like Root Explorer and delete them.