Go to MyFarm >Locations> Edit Location > Fields

Select the field you want to map.  Click 'add boundary'

Click on the map where you want to start mapping.   Every time you click you get a dot, click to get the shape you want.  

Each dot can be dragged around so you can get as detailed of a shape as you want

If you have sub-fields, decide if you want to map out just the main or parent fields, or the sub or child fields.   We suggest you don't do both as the overlapping can make the map confusing.