I typed "Fundamentals Of Rifle Marksmanship" into my browser.
Lots of info under that heading on the Net, including videos.
Not as good as personal instruction but should help with the basics.

I would practice from standing, sitting and prone. I've shot deer from blinds with the rifle rested on the sill of the firing port, from standing, (several) and one from prone. It was my second deer of the day. The first was taken from standing.

The AR in .223 would not be my first choice for deer, but I think it would work with deer bullets (softpoints), careful shooting at short ranges, with good shot placement.

Many of the deer I've killed were shot at 50 yards. Some were under that. The longest kill was made at just over 200 yards. With the .223
I'd try to get shots no longer than 100 yards.

I plan to shoot deer standing broadside and place the bullet just behind the front leg. I aim for the lung area and sometimes hit the top of
the heart, or the arteries above it. With that shot I have not had to
trail many deer. Most have fallen on the spot.

It's good that you are practicing ahead of the season.