To create a custom field, you must first be in the Add Wine flow.
On both the Mobile App and the Web App, at the bottom of screen, select the field that says + Add Custom Field. Then click on the gold + Add button on the right low corner. The create Collection Field window will open and you then then type in the name you want to call it (e.g., Dosage) and then choose if this is a Text Field, Number Fields, or Date Field. Once done, SAVE your selection.