Learn Polish: Kawa jest już gotowa, musisz tylko dolać śmietanki. - The coffee is already ready, you just need to add some cream.

The verb dolać is an aspectual verb of addition. In Polish, verbs that describe adding a quantity of an uncountable substance, especially liquids or powders, take the partitive genitive instead of the accusative. This genitive implies you are adding some cream, rather than an entire discrete object.

What is the exact difference between lać and dolać?

The base verb lać just means to pour. Adding the prefix do- creates dolać, which specifically means to add liquid to something that already exists, or to top it up. It perfectly captures pouring cream into a cup of coffee that is already prepared.

I know śmietana means cream. Why is the diminutive form śmietanki used here?

In Polish, śmietana almost always refers to sour cream, which you wouldn't want in your coffee! Śmietanka is sweet cream or half-and-half specifically used for coffee or desserts. So here it is not just a cute diminutive, but a completely different ingredient.

Could I say tylko musisz dolać instead of musisz tylko dolać?

You could, but musisz tylko dolać is much more natural. The word tylko (only/just) typically sits right before the word it emphasizes. Here it highlights the single remaining action, which is dolać (to add by pouring), so placing it immediately before that verb sounds best.