Choose a 12" x 12" piece of scrapbook paper. Lighter colors work well for writing right on the envelope. If you use a darker color, you will need to write the address on a white address sticker.
Other supplies you will need are scissors, adhesive, paper cutter, and a stylus (or popsicle stick). Optional supplies include a corner rounder.
Score your paper at 3" and 8.5" going one way. Turn the paper the other way and score at 1.5" and 10.5". Since I don't have the fancy score thingy by Martha Stewart (ahem, Santa) I just use my paper cutter and my stylus. You can use a popsicle stick for scoring as well. Or even more novel, use a scoring blade if your cutter has one.
Fold on all the score lines. Fold over the largest sides - with the 3" piece on the bottom (or first). Put adhesive on the edge of the wider piece (3.5" side) and fold that one down too. I use double-sided tape and it has worked well in the past. Use whatever permanent adhesive you like.
Now you will have two short sides still unfolded. I like to cut off 1/2" of the next side that will be adhered. You don't really have to, though.
Before sealing the third side, I like to use the corner rounder to snip the four corners that will show when the envelope is closed.
Seal the third side with your favorite adhesive. Again, put it next to the edge to make sure it doesn't get snagged during mailing.
Ta-da! What a cute envelope. Think about how much more cheerful the recipient will be getting mail in a fun envelope! Think about how cheerful the mail people will be when they handle such cuteness.
Now just fill your envelope and use the adhesive to shut the fourth flap.
Another way to treat the corners is to angle the edges. I usually just eyeball this, but you can be more exact if you want. Measure about 1/2" in from the corner and cut diagonally toward the fold/opening of the envelope.
Go ahead, make the mail a little more fun!