That is correct - you should download goo manager, and then, Hit Menu button and select Install OpenRecoveryScript.
That should install the current version of TWRP (184.108.40.206 for your device I think).
(I assume you prefer TWRP. Else, you can install CWM instead - but I haven't looked at the steps to do that. I use TWRP. Regardless, the install should be seamless).
And yes, you definitely want to create a nandroid backup before you do any modifications to system files (and you need to install TWRP or CWM to create nandroid backups)
Note: The Nandroid backup of the stock Sprint ROM is sizeable. Remember to not change the file/folder names after you create the backup, and it may be a good idea to compress it.
I chose to backup to the 64GB microSD card (TWRP lets you do that) because I didn't want to lose 2-3GB of internal storage to the nandroid.
20gb free cloud storage with this link