Easy Shikaku boards are built so that at every moment some clue has exactly one rectangle that fits, or some empty cell can only be reached by one clue. Lots of small numbers — 1s, 2s, 3s — mean lots of quick, certain moves, and the board falls in a satisfying cascade.
That guarantee is graded by our solver, not eyeballed: a board only ships as easy if it's solvable start to finish with singles and forced rectangles alone. If you find yourself guessing on an easy board, re-scan the edges — the forced move is there.
Start with 5 × 5 or 7 × 7 grids, and read the how-to-play guide for the two techniques these boards teach.