First add a measured margin around the inside of a field
Select the field on the map
Select “Add Margin” from the tool bar on the map
Add the margin width
Click "Apply"
Select the margin on the side bar and then select the slice tool
Next zoom into the corner of the field where the fieldmargin cut-off
Use the slice tool to cut through that section
Next draw a line using the slice tool to the other corner of the field where the margin ends
Slice through this section
Double click to make the cut
Next use the merge tool to merge together the main field section and the two sides of the field that do not have a margin
Select Merge
Tap the fields on the map or on the side bar
Tap apply
You can then change the name of the margin on the side bar.
Click "Save"