Syncing Local FIles to my iPhone

[IHaveAMissle]

I have an iPhone 5s and am having difficulties syncing local files from my computer to it. I have Spotify Premium and everything but it just always gives me difficulties.

I ended up just putting them on my actual music app but I want them to be on Spotify just for the convenience of it.

All of the different posts that I have looked at have issues that they found that worked when they changed but I have all of those correct but still no syncing.

If you know how to help let me know!