Use objects marked with lightning  (Generators) to add new items to the field, and merge items until you get the right one.    If there is already an item required for the quest on the field, it will be marked with a green checkmark  .

Two identical Generators can be merged into one higher-level Generator, just like regular items. To add a new item to the field, double-tap a Generator of your choice. It takes 1 energy point to generate a new item. Note that you cannot choose which item will appear: they are generated randomly from a pool of possible items.  To learn which items can be produced by a Generator, tap on it once and then tap this icon    on the right of the field. There, you can also learn the level of the Generator.