Learn Finnish: Miten valitset oikean sävyn? - How do you choose the right shade?

Yes, you could say Kuinka valitset oikean sävyn? Both miten and kuinka can mean "how" or "in what way" when asking about the method of doing something. Miten is a bit more common in everyday spoken Finnish, while kuinka can sound slightly more formal, but they are fully interchangeable here.

How is the verb valita conjugated to get valitset?

Valita (to choose) is a Type 5 verb, meaning it ends in -ita or -itä. To conjugate these verbs, you replace the final -a or -ä with -se- before adding the personal ending. So, the stem is valitse- and adding the "you" ending -t gives us valitset.

Why do oikean and sävyn end in -n instead of being in the partitive case?

They are in the accusative case (which looks exactly like the genitive case in the singular). This case is used here because the action of choosing is complete and affects a whole object. You are picking out one specific, entire shade rather than an ongoing or indefinite amount, which would require the partitive form oikeaa sävyä.