Within, what your body can deal
with, try the following cadence to establish your timing.
For a 5 step, try slow, slow, quicker, quicker, quickest. If you push-away after the first step or simply unhinged on the second step, it should seem as your ball passes your right leg going back, you will be walking away from your back-swing in the opposite direction.
The result is you will finish your slide and your ball will still be behind you. Here's where it get's tricky. You must wait for your ball to swing forward without any help from you.
Because you're already stopped, you become the fulcrum point of your arm swing. As the ball is reaching the bottom of your swing, this is where your thumb should exit. Only now can you accelerate your fingers through the end of your delivery.
Don't 'lift' with your fingers, instead allow your wrist to remain relaxed so that your fingers create a 'whip' like effect, with as little effort from you as possible.
From the time you come to the end of your slide, until the ball reaches the bottom, is only about a second of maybe 1.5 seconds based on back swing. Waiting for that ball to fall to the proper position is the hardest part.
If you try to assist your swing in any way, your torso will turn slightly left wiping out your body angle. Because of the turn of your torso, your shoulders will also close, your arm will begin to arc away from your swing path, your elbow will get too far outside and you either go around the ball or pull it across your target line.
Be patient, stay still and only apply the whip action from your wrist after your thumb exits the ball. What ever you add to your ball should not change the arc of your swing or the direction of your swing plane.
If you get an opportunity to watch any pros warming up, you will see they start by taking a single step or maybe 2, and allowing the ball to swing forward without any help. The more you do it, the easier it becomes.