This envelope is for the 5 inch square card I posted yesterday.
Cut a square of Kraft paper (I used a manilla suspension file) & score as shown below.
Round all the corners of the square & one corner of the lining piece. Use temporary adhesive to stick the lining piece in position on one of the envelope flaps. Draw lines parallel to the Kraft piece on each side, & fold the flap over.
Remove the liner & cut along the lines you have drawn. Stick the liner down to the flap first, then fold over & stick the inside.
Fold & stick the 3 other flaps, ensuring there is no adhesive on the inside of the envelope.