How do I create a stationery in mail?

You can't edit existing stationary layouts in Mail. But you can create your own as this exerpt from Personalize emails with stationery in Mail on Mac - Apple Support explains:

You can also create your own stationery template. In a new message, format text and add images, then choose File > Save as Stationery. Your stationery appears in the Custom category.

pimentel wrote:

How do I edit a stationery in mail? Or How Can I create one that can Be Use in Mail?

Personalize emails with stationery in Mail on Mac - Apple Support provides an explanation.

Existing stationary layouts can't be edited.