You can define it on field in xml like this:

<field name="your_field" attrs="{'readonly': [['state', 'not in, ['draft']]]}"/>

From the interface, you can enter developper mode (upper right corner drop down uder the user name - must be admin, abot odoo, clieck on developper mode) then edit the view - good for testing. The issue with this approach is that when the module is updated your modification will disappear. 

If you want to go further search the documentations for view inheritance.