How to use Pioneer FLX4 Smart Fader with Serato DJ Pro?

I have one track in channel 1 sitting at 100 BPM, and wanto use SMart Fader to mix into Channel 2 which has a track at 120BPM

How is this done using Serator DJ Pro?

Do I need to have both channels on SYNC in Serato, or do I leave the Sync off?....... or are there any other settings I need to ensure to get it to work?

So far ive tried both with Seratoi SYNC on and off on both channels and cannot get it to work

Any help appreciated

Thank you