Here, well-defined means that any given object must either be an element of the set, or not be an element of the set. Memorize: We say sets A and B are equal, and write A = B if x ∈ A ⇔ x ∈ B (that is, have exactly the same elements).
It typically involves (1) plowing to "till" or dig-up, mix, and overturn the soil; (2) harrowing to break the soil clods into smaller mass and incorporate plant residue, and (3) leveling the field. Initial land preparation begins after your last harvest or during fallow period.