To setup Checklist for Salesforce in classic, you need to create a button and add the button to the Page Layout.
As the first step, create the button for the object.
We already created buttons for Account, Opportunity, Lead, Contact, and Case which you could use as an example.
Then, add the button to the Page Layout:
Add the button to the Layout
- for the Classic UI from Buttons to the Custom buttons area (green)
- for the Lightning UI from the Mobile and Lightning Actions to its area (yellow)
For the Custom Object, make sure that Allow Activities is selected in the Custom Object Definition.
(Otherwise you may receive the error message: Related To: Related To ID: id value of incorrect type: ... or )