Find A trail that they are using, sit close enough for A shot, and hold really still. The deer will show up.
Then you and your 870 do the rest.
Don't over complicate it, don't put unneeded pressure on yourself. Look for the first legal deer that presents you a good shot, and take it.
20 acres isn't big. One walk through there could run everything out. Because of that I'd be careful where you set up and how you get to your spot.
Good luck. Let us know when you get one!
-Jake