Where to compose email body

I want to trigger an email to the customer after he ordered a product. How can it be done?

I can only find a "Send Email" option in WebAssist in which I see many email templates. However, I can't find any way to compose the body of my email. Where can I do so?

I tried to edit the "Block.php page. I am not sure if this is the way to compose the email. I can't find any form data which I can bind into the email body.

Do you have an instructional video on how to create email body and bind form data? Else it's very difficult to use the email feature. The Universal Email plugin in the past used to be very user friendly. The current version is supposedly better but somehow it is not intuitive.

Look forward to your help.