Thanks for visiting the Apple Support Communities. From your post, I understand that you would like instruction on purchasing and downloading content from the iTunes Store without signing up for an Apple Music membership. I'm happy to help!
Once you are signed in, locate and open the iTunes Store app on your iPhone to make the purchases. This is a separate app from the Music app, and in iOS 11, the icon looks like this: . If you cannot locate it by sight, try searching for it: Use Search on your iPhone, iPad, or iPod touch - Apple Support). Once the iTunes Store app is open, you can use the following information from the iPhone User Guide to purchase and download your music: Purchase, rent, or redeem - iPhone User Guide.