Start each rep from a dead stop, don't bounce them after rep 1.
Take a deep breath and pull your chest up at the start of each pull. Pull the slack out of the bar before you start moving it upwards. Try and make the bar float. Feel the "wave of extension" all the way down to your sacrum. I see some low back rounding that you will want to avoid.
Lockout is a bit exaggerated - just stand up tall with shoulders back. No need to keep going back any further.