When counting outcomes, first decide whether to multiply or add
See the multiplication rule with a tree diagram for picking a menu, and the addition rule with a grid for counting shortest paths. The key to both rules is counting every case once, with nothing missed and nothing double-counted.
The multiplication rule — if one thing can happen m ways, and for each of those, a second thing can happen n ways, then the two things happening one after another can happen m×n ways. If you pick a drink and then pick a dessert, the choices always split into the order "drink first, then dessert," so nothing ever overlaps.
Number of drinks (m)3
Number of desserts (n)3
The addition rule — if two mutually exclusive cases (they can't happen at the same time) can happen m ways and n ways, then either one of them happening can happen m+n ways. The path to any point on a grid always comes either "from the left" or "from above" (never both at once), so the number of paths to that point is the sum of the path counts of its two neighbors.