I know this is a simple, stupid question.

Sync your iPhone, iPad, or iPod touch with iTunes using USB - Apple Support

Just plugging it into your iMac and selecting playlists, artists, albums etc for syncing should do it, unless you have some work-related complication.

steveh46a wrote:

When I click "Manually Manage Music and Videos" and then the "Apply" button, I receive a message that "An iPhone can be synced with only one iTunes library at a time." However, I have to sync via iTunes with my work computer to sync my work calendar with my iPhone calendar. So, is there someway to get the one tune I'm currently trying to transfer to my iPhone without undoing the sync with my work PC? Or do I just have to copy the tune to a flash drive, carry the flash drive to work, copy the tune to my work computer, use iTunes to transfer the tune to my iPhone and then erase the file on my work computer? That seems ridiculously complicated for something that should simple.

An iPhone can be synced with only one iTunes library at a time.

That is the answer. It is most likely to prevent people sharing content using iOS devices. You have the workaround of using removable media. Sorry if that isn't acceptable.

There's a similar situation at https://discussions.apple.com/thread/7514881 but since you do not want to use cloud based services you are restricting yourself into a corner.

If they are iTunes purchases and the phone uses the same AppleID used to make the purchases they are already in iCloud and you just need to re-download them to the phone. If you are using a different AppleID then you need to read the link I provided earlier for workarounds. If you are using the same AppleID and have non iTunes Store purchases then set up an iTunes Match account where non-purchases are also in iCloud.