Paint the wall first.
Once that dries, place painters tape.
Then cover painters tape with another round of the wall paint.
Once that dries... trim over as haphazardly as you wish, staying away from the far edge.
Once THAT dries... remove the painters tape and marvel at your perfectly trimmed barn.
THEN... go back inside your house, take a look around, and motherfuck the shit out of whomever painted it last.