Decide on a co-ordinating paper to match your card. It is up to you if you want to see the patten on the inside or outside of the envelope.
Start by working out how much room you will need for the card and then score and fold the paper to create the 'bottom' of the envelope.
|Decided where to score for a bottom fold|
|Fold the paper to create the bottom fold|
Next you need to work out where you want to create the side flaps, ensure that you leave a bit of extra space around the card. Score and Fold each side of the card.