Mary asked how I managed to line the envelopes for my friend Judy's shower invitations so here is the easier DIY ever. Get an envelope, pretty paper (I used the cover of the Paper Source catalogue), a pencil, scissors and glue stick or glue pen.
Trace your envelope onto the wrong side of the paper. Cut it out.
Slide the paper down until the top looks like it's about right. Trim the bottom of the liner off so it will fit inside your envelope.
Put a little glue on the bottom and slide the liner into the envelope. Glue the flap portion and BOOM! Done. Lamest DIY ever, so effing easy.