You will have a hard time sighting in a springer AR off the bench as the mechanical force of the spring action will always move the rifle POI when you pull the trigger - Newtons Law of Physics every force has an equal force in the opposite direction.
You will have to free-hand your AR sighting in.
I'm no expert but I have a very heavy AR, with a strong spring action and this is what I do.
I found sitting down and having a steady rest for my front arm's elbow the big key.
I lock my off arm elbow to the side of body and shoot the AR across my chest. My body is pointing nearly 90deg from the target.
Place your off hand just in front of the trigger guard so that your arm is vertical.
The rifle sits in the palm of your hand with your finger pointing down the stock towards your target.
You can also rest the rifle on top of a clenched fist to get some more height.
If you want more steadiness, you could try sitting on a stool with you back against a wall or door jam.
Your AIM rises up as you breath in - but do not hold your breath as you squeeze the trigger as then your blood pressure rises slightly and your heart beat will thump harder and also affect you POI.
Be comfortable with a steady and light hold and squeeze the trigger when your ready in a couple of breath cycles.
Don't wait too long as you will then start to anticipate and something will start to change slightly - firm up your grip, pull the rifle into the shoulder more or less, hold your breath etc. You are better off starting your aim again after a few rest breaths.
Shoot 5 shots with the same rhythm and routine.
If you want then try a different routine but shoot 5 shots again. eg change your trigger hand thumb position, trigger hand grip pressure, pull into the shoulder, squeeze the trigger at a different point in your breath cycle etc.
The key is repeatability.
You need to get into a rhythm.
This is especially true when you then go hunting with the AR.
Have Fun Plinking.