Learn Russian: Что вы делали вчера?

Делали is an imperfective verb. In the past tense, imperfective verbs often focus on the process of an action, like "were doing" in English. Here, you are asking about the general activity of the day, not a specific finished result.

How would this sentence change if I wanted to ask about a completed result?

You would use the perfective verb сделали. Asking "Что вы сделали вчера?" means "What did you get done yesterday?" or "What did you accomplish?", whereas делали just asks how the person spent their time.

Why does делали end in -ли?

In the Russian past tense, verbs change their endings to match the subject's gender and number. Because the subject is вы (you, plural or formal), the verb takes the plural past-tense ending -ли.

I keep forgetting, is Что pronounced with a "ch" sound?

No! Even though it is spelled with a Ч (which usually makes a "ch" sound), the word Что is a famous exception. It is pronounced like "Што" (Shto).