Summarize the following:
You may need to use a letter opener or similar tool to help you prise open the stuck down edges of the envelope. Lay it open and flat on a sheet of cardstock.  Choose an envelope of the size you want the gift envelope to be. It's a good idea to have a few envelopes to open up, in case you rip any when trying to create the template. Do so using a pencil, as you only need to follow the lines and don't want the design marring the end result. Note that for the sake of clear demonstration, this article's example has used a black marker. Once you've drawn the outline onto the cardstock,  keep the template for use later in these steps. It's also useful to keep it if you intend on making more gift envelopes for other people. After removal, draw the inner fold lines for the envelope. These are the lines that form the inside of the envelope, its inner rectangle shape. See the image accompanying this step for clarity. Drawing the inner lines can be skipped if you have a steady eye for keeping a line as you score (indent mark) the rectangle in the following step. It is recommended though that you add the lines if you're a beginner or find it hard to keep straight lines when drawing or cutting. Scoring here means to make a small "almost" cut that is actually an indentation. This is made to aid with making the folds on the stiff card. Do the scoring using scissors; gently run the tip of one of the scissor blades down each fold line. It is important to do this lightly as you're only making an indentation, not cutting the card.  Once you've scored the rectangle, gently bend to create the folds of the envelope. This isn't needed for the gift wrapping outer part of the envelope because gift wrapping is far more flexible than the cardstock. Go slowly and carefully to ensure the cut is even and neat. Discard the edge pieces (or keep as scrap paper for reuse in craft projects).

Summary:
Gently pull apart a standard envelope. Draw the outline of the standard envelope onto the cardstock. Remove the template envelope. Score the fold lines that you drew in the prior step. Cut out the external edges of the envelope following the template lines drawing on the cardstock.