I encased the ATC in between two layers of sheet protector and sealed it with an iron (between two Teflon ironing sheets!) Then I stamped white card stock with blue snowflakes. I folded the card and cut the window out. I glued the sheet protector package to the inside of the card fold, sprayed adhesive on the inside and folded the card. This is designed for the ATC to be removable. This is the front of the card (obviously!)
This is the inside. The writing you see is on the back of the ATC.
Then I made printed a white cardstock page with this snowflake pattern, cut out the envelope form, folded and glued the edges. Then the card was placed in the envelope and it is ready for the mail! I covered the mailing label just so the address would not be posted.