CDJ 900 Nexus - Beat Sync function

Hi - I've had a pair of CDJ 900 Nexus for c.10 years now, but never used (or even tried to use) the beat sync function.

If I wanted to now explore this, what tech would I need to faciliate this, please?

My CDJs are hooked up to an old DJM 600 mixer.

Many thanks in advance.