I often see people sighting-in a rifle with no rear bag, and trying to hold the rear of the rifle steady with their hand, like this.

That simply will not result in a good, steady position.

Get a rear bag and learn how to use it.

I place the rifle butt between the "ears" of the bag and then use my left hand to position the bag and to squeeze it to get the sights on target.

One of the biggest mistakes I see people make is that they are often confused about where to place their left hand.

Some want to place it under the fore end of the stock like this.

Don't do that.

It will only open-up your groups.

Some folks want to place their hand over the fore end, especially if using a scope (as you can't see irons sights doing this)

Don't do that.

I shoot off of concrete benches, and they can be hard on the old elbows.

I place a folded towel under my right elbow like this.