How do you print a single envelope for a contact?
Create a new envelope PDF format that corresponds to your printer's settings at Administer CiviCRM » Communications » Print Page (PDF) Formats
Create a new mailing template from Mailings menu » Message Templates.  Fill in the template with the appropriate tokens you'd like - and also a return address if applicable.  Select the envelope PDF Format that you just created.
When viewing a contact, press the "Actions" button below their name and select "New PDF Letter", and select the envelope template and print.


This video tutorial covers a similar workflow, but for sending envelopes to a larger group of folks: https://www.youtube.com/watch?v=i00269Y7pB4