When you select a menu item in the Toast POS app that has portions enabled, you see a Portions button with the modifier group buttons:

Tap the Portions button to access the portion options, in this case, 1st Half and 2nd Half:

Tap a portion option, then select the modifier options you want for that portion. Tap Done when you are finished specifying the modifier options for this portion.

Repeat this process to specify the modifier options for the other portions. Once the modifier options have been defined for all the portions, you should see them under the menu item on the order screen: