Of course I needed an envelope to go with my birthday cards and found a free template to use. The size of the card dictates the size of the envelope. My card was too big for an envelope cut from an A4 piece of paper, so after a bit of trial and error, I successfully cut one the right size from 2 A4 pieces. I managed to join the pieces more or less at the fold of the base of the envelope, and I added perforated lines at the folds to make folding easier.
The end product looked pretty good!