シ/ツ and ソ/ン are the pairs that keep tripping people up years later. The difference is which way the long stroke goes — learn that once and both pairs are solved.
shiシ
tsuツ
shi / tsu シ: the two short strokes lie flat and the long one sweeps up from the bottom-left. ツ: the short strokes stand up and the long one comes down from the top-right.
シ
ツ
soソ
nン
so / n ソ's long stroke comes down from the top. ン's sweeps up from the bottom. Same test as シ/ツ.
ソ
ン
kuク
waワ
ku / wa ワ is closed along the top like a roof. ク is open at the top-left.
ク
ワ
uウ
waワ
u / wa ウ has a short stroke standing on the roof. ワ does not.