Try to open your envy by trimming the edge rather than ripping it open like above (c/o my husband - my opened envies always look neater) if you want to use the whole envelope.
If not, it's still salvageable - just trim off the jagged edge. Then cut off whatever you don't need from envelope.
Open envelope to get it ready to add a tab on side.
Run envelope and circle tag tab through sewing machine.
Then sew top and bottom edge to close off envelope. Leave right edge open so that you can slip a piece of cardstock or photo into your new envelope. Alternatively, you can go back to the first step and turn your envelope inside out if you don't want the printing on the outside (before you start any sewing).
I made a whole bunch of these for the Love You art journals. Instead of sewing the top and bottom edge of each one individually, I sewed all the top edges and then all the bottom edges giving me this long envelope banner.