You can vary the cuts if you wish but here is a basic method.
Wash your hands. You'll be handling the ground meat so start with clean hands to avoid contamination
Cut the meat into 1 1/2" cubes. You don't need to use a micrometer but keep the pieces uniform in size so you end up with a more uniform end product.
Place the chuck in your food processor fitted with the metal blade. Use short pulses that last about 1- 2 seconds per pulse. *Pulse 10 times. Do not use the "auto" button, only the pulse.
Remove the chuck and place it in a bowl or on a plate. Now repeat the process with the sirloin. Add the sirloin to the chuck and mix well with your hands.
*Pulse about 6 pulses for a coarse grind for making chili.