If you want to import your own html template and use it for your email template, then you can simply follow these steps:
Create your campaign: add name and recipients
Add sender details, subject, and other settings if you want
Select "custom html" then choose the template
Go to drag & drop editor
Copy and paste your html code on the bloc html
Important note: You need to remove the </body>,</html> ,<html> and anything in <body: something>, these tags should not be copied.
6. You can adjust the width of the container by clicking here
7. Save and continue