Do you want to re-format the old HDD to remove windows and use it as a pure storage drive? Or do you want to simply point "My Documents", "My Music" etc to the other drive?
I'll answer both. If you want a simple storage drive with windows removed first I suggest backing up important files on the SSD or flash drive (leaving big games that can be re-installed, dont forget save files!), then using windows diskmgmt.msc to format the 2nd drive, and create a new partition volume for files, then move your stuff back (type diskmgmt.msc in windows search bar to access).
If you just want to re-direct your folders to 2nd drive, in the file explorer, right click "Documents" on left side, go to "Properties", and delete the current directories pointing to the C: drive, then add new ones that point to the "2nd" drive.