1. In the ingredient's Modifier section click Add A Different Ingredient or Sub-Recipe Substitute.
2. In the picker (same filters and search as the ingredient picker; the ingredient itself is excluded) click + on each substitute.
3. Each substitute appears as its own panel under the ingredient. Use the toggle next to its name to make it available to customers, enter the quantity of the substitute for every portion size, and the price change per portion (with a Copy ... price to all portions shortcut).
4. Save the meal.

Substitutes with quantity 0 on a portion, or on inactive portions, are dropped on save. Substitutes already saved on the ingredient itself (in the Ingredients area) are pulled in and listed automatically when you open the meal. Customers who can swap that ingredient then see a Swap button next to it in the order/meal-info window, which opens a dropdown of the available substitutes (with any price change shown).