Earlier I showed a couple of envelopes I made. But, maybe you're not into starting from scratch. Okay, that's fine. How about repurposing some of your junk mail? Anyone up for that? Take one envelope. This one came with my phone bill. It's the one they want me to use to send my payment back in. I also use the envelope the bill itself comes in. Open it carefully at the seams.
Fold the envelope wrong side out. Many of these junk envelopes are the security types with cool patterns inside. Apply glue to the flaps and fold them in. Notice the flaps are on the outside of the envelope this way, rather than on the inside.
Decorate the envelope if you choose. Now it's ready to address. You can slap a label onto it and it's good to go. Or, you can write the addy on plain paper and tape to the inside of the window. Write me a letter. Maybe my reply will come to you in one of these!