Under the 'Marketing' tab, select 'Emails'. From here click the green 'Create Email' button in the righthand corner.
Under 'Basic Info' you will name your email and assign it to a campaign (recommended). Under 'Template' you can choose to design a brand new template using one of the pre-populated templates provided, or use one of the templates you already designed by clicking 'Your Templates'.
Make sure the email looks the way you want under the 'Design' tab and make any necessary edits.
Under 'Setup' select the sender email address, the email address for replies to go to, the email subject, and the sender's name.
You will also select the list you want to send the email to, and any list you do not want this email to go to (suppression list.)
Preview the email you have created by clicking the 'Confirm' tab and either save as a draft or send the email immediately.