Just for fun I’ve been making envelopes from wallpaper samples.  I have a plastic template I got years ago from a craft store, but you could easily use a printable template.  If you search Google Images for “envelope template“, you can choose from a variety of envelope shapes.  Print the template, cut it out, trace it on your paper, and cut out your envelope.  Use a scoring tool to make your folds smooth, and use glue or tape runner to adhere the sides together.

Here’s an idea for making fabric envelopes I’d eventually like to try.  Do you have other envelope making ideas?