Start off with an envelope in the size you want, you're going to make a template of this that you can use over again and again.
The great thing about this, is that you can make any size or shape envelope that you want, so you'll always have the right envelope in stock.
Fold out the envelope, carefully unsticking it at the edges, don't worry you can re-stick it later.
Lay the folded out envelope flat on top of a sheet of heavy card. Trace around the outline with a pencil.
Cut out the card envelope shape and then draw a line (with a ruler) between all four corners, so that you get a rectangle in the middle.
Use the ruler and a cutting knife to score lightly along these lines.
You can now fold your template up in to the envelope shape.
Now take the ruler and measure half a centimeter in from the edge, marking points around the outline.
Draw in a second, inner, outline half a centimeter in from the edge.
Use the cutting knife, so cut small slits along this template, making sure to leave the edges un-cut (otherwise it will just fall apart).
You should now have a template that looks like this.
Now comes the fun part. Pick some paper you want to make envelopes out of. You can use magazine pages, wrapping paper or anything you like.
I used an advert for the Zune for the ones shown.
Take your template and draw around the outline on to your paper.
Cut this out and score along the edges like before.
Take some tissue paper and draw along the inner outline on to this.
If you're going to be sending something delicate, you could cut bubble wrap to line your envelope.
Cut the tissue paper out and glue on to the envelope, so that the tissue paper will be on the inside of the envelope.
Fold the envelope up, glue down the bottom edges of the envelope and you're all done!