First, download TunesKit Apple Music to MP3 Converter and please guarantee that you've authorized your computer to stream Apple Music songs. Then launch TunesKit and from the main interface, you will find the Load iTunes Library button to add your Apple Music songs. Or directly drag files from the Apple Music media folder to the TunesKit.
When you finish step 1, open the Format panel to select an output format for Apple Music songs. Serato DJ Pro supports file types including MP3, OGG, ALAC,(Mac & Windows 10 only), FLAC, WAV, MP3, MP4, M4A, and AAC(Mac only). TunesKit allows you to convert any audio to MP3, AAC, M4A, M4B, WAV, and FLAC, which means that you can choose any one of them as your output format. We recommend MP3.
Once you are done with all the settings and editing, you can press the "Convert" button. Wait for a few minutes until the conversion finishes, then you can locate the unprotected Apple Music files in the folder you've selected. If you don't remember which folder you choose, you can go to the Converted History to find them. Since you get the converted Apple Music tracks, you can import them to Serato DJ for remixing.
For more: https://www.tuneskit.com/apple-music-tips/mix-apple-music-with-serato-dj.html