Cut a piece of paper just slightly narrower than the width of your envelope, so that you can easily slip it inside
Turn the envelope over with the paper in it and draw around the envelope flap.
Cut along the line you have drawn and then remove a small amount from the bottom of the paper. (Approximately the width of the glue strip on the envelope
Slip the paper back into the envelope and glue or tape into place.
(I generally only glue the top section to the flap,)
And there you have it - a lined envelope !