The key point when making a bento is to arrange the ingredients so that it's three-dimensional.
The important thing to remember when making a bento is to not lay everything down flat.
「弁当の盛り付けポイントは立体的に作ることです」= The key point when making a bento is to arrange the ingredients so that it's three-dimensional. / The important thing to remember when making a bento is to not lay everything down flat.
ボキャブラリー
key point/important thing = ポイント
when = ~の時
arrange = 盛り付ける
ingredients = 材料、おかず
three-dimensional / not flat = 立体的
remember = 覚える
lay = 置く、盛り付ける
※ 「bento」はそのまま通じると思いますが、代わりにに「Japanese packed-lunch」や「Japanese lunch box」という言い方も使えます。