Learn Farsi: شما دیروز چقدر آرد خریدید؟

No extra words are needed! You simply place cheghadr (how much) right before the uncountable noun ard (flour). They sit side-by-side: cheghadr ard.

Can I put cheghadr at the very beginning of the sentence?

It is much more natural to keep it where it is. In Farsi, the subject shoma and the time word dirooz (yesterday) usually come at the beginning of the sentence. The question phrase cheghadr ard stays close to the verb at the end.

Could I use cheghadr to ask how many apples someone bought?

No, because apples are countable. cheghadr means 'how much' and is used for uncountable nouns like ard (flour), water, or money, or to ask about verbs. For countable things, you must use chand or chand ta (how many).

How do we know who bought the flour just by looking at the verb kharidid?

The past stem of this verb is kharid (bought). The -id at the very end is the personal ending that matches the subject shoma (you, formal or plural).