Final answer:
To create a custom slide layout, John needs to first modify the existing slide layout by changing the position and size of the placeholders, formatting the text, and customizing the background and theme. Adding new placeholders and adjusting text indents can be done afterwards, while deleting the content placeholder is not necessary.
Step-by-step explanation:
To create a custom slide layout in a presentation, John needs to first modify the existing slide layout. This can be done by changing the position and size of the placeholders, formatting the text, and customizing the background and theme of the slide layout.
Once John has modified the existing slide layout according to his needs, he can then add new placeholders for pictures or other content if required. Adjusting the text indents in the content placeholder can also be done after modifying the slide layout.
However, deleting the content placeholder is not necessary to create a custom slide layout, as it can still be used to insert and format text or other content.