First of all, update to the latest version of Scratch Live (version 2.1 or later). For information about how to update Scratch Live and the necessary operating environment, please visit the Serato Audio Research website. 

Also, update the firmware of your CDJ-900 to the latest version (version 3.10 or later). For information about how to update the firmware of your CDJ-900, please visit the AlphaTheta support page.

In addition to Scratch Live and a CDJ-900, you will need one USB cable per CDJ-900 player.

You will also need an interface for Scratch Live, such as the SL1 or SL3 made by Rane.

Using a CDJ-900:

Connect the CDJ-900 to your computer with a USB cable and start up Scratch Live.

Once you press the CDJ-900's LINK button, the message PUSH TO CONNECT will appear on the CDJ-900's display; press the rotary encoder. Next the message CONNECTED is displayed with DECK:1 indicated; turn the rotary encoder to select the control deck (Using a Rane SL1:DECK:1 ～ DECK:2, Using a Rane SL3:DECK:1 ～ DECK:3) and then press the encoder. That's all there is to enable control with a CDJ-900.