Learn Italian: Sottolineando le cose importanti, non dimentichi niente. - By underlining the important things, you do not forget anything.

In Italian, the present gerund (words ending in -ando or -endo) is often used to show how or by what means something is done. You don't need a separate preposition; sottolineando by itself translates perfectly to "by underlining".

Who is doing the underlining in this sentence?

The subject of a gerund in Italian is almost always the same as the subject of the main verb. Because the main verb dimentichi is in the "you" (tu) form, we automatically know that "you" are the one doing the underlining.

Why does dimentichi have an "h" in it if the infinitive is dimenticare?

It's a spelling rule to keep the pronunciation consistent. Verbs ending in -care have a hard "c" (like a "k" sound). To keep that hard sound when adding an -i for the tu form, you must add an h. Without it, the "c" before the "i" would make a soft "ch" sound.

Why does the sentence use both non and niente?

Italian uses double negatives! Whenever a negative word like niente (nothing/anything) comes after the verb, you must put non before the verb. So non dimentichi niente literally translates to "you don't forget nothing," which is the standard way to say "you don't forget anything" in Italian.