Reallocating files on Dual Drive after installation

Hi all,

There has been extensive information on how to install an SSD and using your old HDD as an accessory storage unit using the optical bay. My question is as follows:

After cloning my HDD onto my SSD (via Carbon Copy Cloner) and using my SSD as my startup disk successfully so far, I want to avoid redundancy and free up space on my smaller SSD. If I were to erase my HDD, how would I move files on my SSD to HDD? Further, how do I make it so that certain files I download from the interwebs is stored on the HDD as opposed to the SSD?

Thank you so much for your help.