Bench Rest shooting an Airgun is about the easiest thing you can do with a Rifle.
If you are shooting a Spring Powered Gun you must use what is called the "Artillery Hold." This hold allows the gun to recoil freely which in turn allows the pellet to escape the barrel before any upward movement takes place. All gun movement is strait back.
The Artillery Hold is best accomplished by supporting the gun's Fore End on a Rest like a 6x6 with a towel covering it. The gun sits in line with the block. This allows the gun to move strait back during recoil and you will have a very loose hold on the gun so it can't get away from you, but is free to move backwards after the trigger is tripped. It will only move about 1" and it isn't going to hurt you in any way. This way you are having the least amount of influence on the gun and the gun is doing what it would normally do if it was shooting itself.
Obviously you will have to aim the gun at a target. In order to get the smallest possible groups out of any gun/ammo combo you must index the sights on the exact same place on the target the exact same way for every shot. This becomes easier when you use a specific type of target.
That is a Diamond Shaped Target Spot.
When using a Scope with Cross Hairs. you will index/split the Vertical Cross Hair on the bottom and top point of the diamond. The Horizontal Cross hair will be indexed directly under the lowest point of the diamond. This is the easiest and most consistent method of indexing your optic on a target there is.
Next using a Red Dot you will index the Top of the Dot on the bottom point of the diamond while trying to keep the dot centered side to side as close as you can. This is obviously less precise than the Cross Hairs but still viable method as Red Dot Sights are more about Target Acquisition Speed than pinpoint accuracy.
Lastly if you are using Iron Sights you will index the top of the Front Sight on the bottom point of the target and split the width of the front sight with the point of the diamond. Your eye will automatically Center the front sight in the Aperture of the rear sight There is no need to try to do it yourself as you will only fail. In fact just ignore the rear sight and put the front sight where it needs to be and let fly!
Open sights are a little more complicated, You must make sight alignment of the front and rear sight so that the front sight is centered in the rear sight and even with the top of the rear sight. Then you must hold that sight alignment with your cheek weld, and then place the front sight on the target in the previously designated position. Then pull the trigger without disturbing any of this.
That's all there is to it, except you've got to practice this every time you shoot and it matters not if you are shooting off a bench or standing on your two hind legs, or hanging from a tree!, Everything is still the same you just have to compensate for your own movement it ain't the guns fault, because you sighted it in on the bench and you know exactly where it will shoot you just have to make it shoot there !
I was taught this by a prominent Gun Writer.. He shoots more in one week than most of us will shoot in our lives.. I asked him how he could get a .44 Magnum Lever Action Rifle to shoot <1" groups at 100 yards with iron sights. The above is what he told me! It works,,, you just have to work at it a little and you'll see the results right away.
Any gun will shoot where ever it shoots. It is up to you to aim it at the same place every time. Only then will you know how well the gun "actually" shoots.
Hope this helps.
Randy