You go to the "Other Information" section on their main / person page. Click on "Add Information" and you will find a link headed "Cremation", from where you can enter the details.
To highlight the fact they were cremated you could add a Note in the Collaboration section or even enter the fact in the "Reason This Information Is Correct:" box, found after clicking on "Edit Death". Incidentally, the latter is the only section on the person page where you can leave a visible statement without having to add a date or place. I use it as a workaround for entering information I want to have highlighted that relates to the broader issues (e.g. cremation) surrounding the death event.
In the example below, I have no further details of the actual death, but can still clearly highlight the fact the the cremation details are "hidden" off-page.
Extract of part of person page where cremation record can be highlighted: