Click the gear icon
Click Setup
Search "Object Manager"
Click Object Manager
Search "Intake"
Click Intake Form
Click Fields & Relationships
Click New
Select a field type, then click next
Fill in required fields for the field type, then click next
Make sure System Administrator is checked, then click next
Make sure the the page layout for the field is selected, then click Save
The custom field will now appear in the layout, but you might want to move it around. To do that, Click Page Layouts..
Click a page layout
Now you can drag and drop the field to where ever you want.
Click Save when you are done.