I added a tee and a ball valve to my return pump. When it's time for a water change, I close the valve to the tank, and open the ball valve (with a short hose), and let the pump push the water into my bucket.
After adding the new salt water, I let the pump just contune to run back into the sump, letting it mix with the sump water for 1/2 hour, then slowly reopen the valve to the DT.
When temp, PH are all stable, then close the ball valve, and I'm done.