This guide is a how-to for streaming using a 3rd party software such as OBS, and many of these instructions may also be applied to other software.
If you are using a DJM, the instructions will be different, please see the DJM guide here.
Step 1 - Update
Ensure that you are running the latest version of rekordbox, that your drivers and firmware are up-to-date, and that your broadcast software is current as well.
Step 2 - Configure rekordbox Outputs
With your DDJ connected and powered-on, open rekordbox, and click the PC Master Output to enable the dual-output mode. This will send the master output both through your DDJ's master out as well as to a secondary output within your PC.
A blue icon indicates the PC Master Output is enabled.
Open the Preferences > Audio > Output Devices and confirm the devices that are selected for the output. Typically, the default is your DDJ and the Windows default sound device, although you can change it here if needed.
Click the X to close the Preferences window.
Step 3 - Configure OBS for Audio
Minimize rekordbox and launch OBS.
Click the + button at the bottom of the Sources window to add a new Audio Output Capture source.
You can keep the defaults, click OK.
Select the device from the drop-down to match the one previously verified in rekordbox.
If you're only streaming audio, you can skip ahead to Step 5.
Step 4 - Configure OBS for Video
If you want to stream a capture of your screen or video input (or both) you can add these as well.
Click the + button at the bottom of the Sources window to add a new Display Capture source, you can keep the defaults and click OK.
If you have multiple displays, select the output display you want to use.
Please note that the Window Capture function won't work to stream just the rekordbox window because of how the rekordbox display engine draws the waveforms. If you use this option, the upper portion of your rekordbox display will broadcast as blank.
If you are showing your desktop, as opposed to a video input source, you should match your input size. Open Settings > Video and change the Base (Canvas) size to match the resolution of your display.
The Output (Scaled) size should typically be 720 or 1080 to keep a down-sampled resolution that is friendly for your broadcast.
You can also (optionally) add a webcam or other video input and configure it here. If you require further instruction on this, try Google.
Step 5 - Broadcast
To make sure you're previewing the audio (and display, if selected), click the Studio Mode button twice (the first time it will toggle to a single-view mode). You will now see audio level in the meter at the bottom, and a video preview (if applicable).
Be sure to adjust the levels on the mixer for any other audio sources; mute the other inputs if they're not being used.
That's it, you're ready to go - simply configure your output stream, or record to a local file.