Learn Russian: Он поступает честно.

Делать translates to "to do" or "to make," focusing on performing a task or producing something. Поступать specifically means "to act" in the sense of behaving, conducting oneself, or making a moral or practical choice in a situation.

How is честно pronounced? Do I actually say the т?

The т is completely silent! You pronounce the word as if it were written чесно. In Russian, when certain consonant clusters like -стн- appear together, the middle consonant is often dropped in speech to make it easier to say.

Why does честно end in -о instead of an adjective ending?

Because it is an adverb describing how the action is done. To describe a verb, you take the adjective form (честный - honest), drop the ending, and add -о to create the adverb (честно - honestly). If you wanted to say "an honest person," you would use the adjective: честный человек.

I've seen поступать used in a different context. Does it have another common meaning?

Yes! Besides "to act" or "to behave," поступать is the standard verb for "to enroll" or "to apply for admission." You will very often hear it when someone is talking about entering a university or school (for example, поступать в университет).