How to generate Duplo instructions?
I can see some Duplo bricks in LDraw parts library, which is used by Stud.io as well, so you should be able to work out instructions the same way you do with usual bricks.
One thing to note is that availability of Duplo parts in LDraw is rather limited.
Alternatively, you can use LDCad (which uses LDraw library) to create your model. In this case you can find most of the Duplo elements under Library sorted by function &gt; Duplo.