Learn Russian: Мама готовит салат.

When talking about food and meals in Russian, you use the verb готовить (to cook or prepare). Even though we say "make a salad" in English, a Russian speaker always "prepares" it.

The translation is "is making". How would I say "Mum makes a salad" as a regular habit?

You would use this exact same sentence! Russian does not have a separate "continuous" tense (like "is making"). The verb готовит can mean "makes" or "is making" depending on the situation.

Because the salad is the object being made, shouldn't it be in the accusative case?

It actually is in the accusative case! The word салат is a masculine noun for an inanimate (non-living) object. In Russian, the accusative form for inanimate masculine nouns looks exactly the same as their dictionary form.