The most obvious point is to select against defects and problems.
Do not breed from any chicken with a crossbeak, or a crooked tail, or crooked toes, or any other thing that would be a defect in all breeds.
Select for general good health: not the chick that needed help to hatch, not the one that needed special care in the brooder, not the chicken that looked extra-miserable when the weather was very cold or very hot or very wet, not the ones that got sick if others remained healthy, and so forth.
If you see a chicken that has very different traits than the rest, it might be an accidental crossbreed, so view it with suspicion too.
Other than that, I would say to select in favor of the traits that made you want that landrace in particular: good foraging ability, or predator awareness, or good mothering ability, or a variety of feather colors, or whatever other traits they are known for.
