Incoming Invoice and Purchase Order Custom Fields

Custom Fields can be created at Purchase Order and at Incoming Invoice level. Go to the Object Manager > click the Menu icon > Setup Custom Fields.

Note: Your User Permission Profile needs the "Server Setup" Permission enabled in order for you to change/add these.

For Purchase Order Custom Fields you can setup different "templates" each containing a different set of fields.
To just use the Default Template, choose "Edit" in the Custom Fields Setup window. [2]
You can leave the "Default" Name or enter a new name [3].
Add the needed Custom Fields through theicon. 

Read more information on the different (Widget) Types of Custom Fields.