How to Reset or Restore 5th and 6th GenerationiPods to Factory settings

I have two iPod Nanos - a 5th and a 6th generation. They do not show up on my list of devices on my Apple ID page but I have been using them since about 2011 - the 5th Gen. I assume however that they are somehow linked to my Apple id someplace deep in the bowels of the iTunes system.

I would like to give them to a friend but do not know how to "reset" or "reformat" them so they would no longer be linked to me i.e., my apple id so my friend could just plug them in to his iTunes account and sync his music to them.

I believe if I just gave them to him and he plugged them in to his computer some sort of alarm would go off - they might shut down or cause some other irreparable harm to the devices, his itunes account, or for that matter even my account . So is there some way to reset them to factory settings and basically just have them ready for him to plug in and use?

I would appreciate any help with this matter as they are very good devices and work flawlessly and I would like to give them away without causing any problems.

Thanks in advance