How do I set the checkboxes on the form to be checked by default?

If you want to have a form where the checkboxes are pre-checked, you can set it up in the Form editor. 

Step 1: Click the pencil icon to edit the Checkbox field:

Step 2: Insert values for each option. Note that the values for the options should be different:

Step 3: List these values in the Default value field (comma separated):

Result: The options on the form will appear as pre-selected: