Envelope lining is one of the easiest ways to transform an ordinary letter to an extraordinary one. There are only three essential things you need for this project, envelope liner templates (you can purchase them from Paper Source, or any craft stores), double sided tape, and beautiful paper. The outcome of the envelopes is based solely on your taste and choice of paper. 

You trace the template on the piece of paper, and you cut it out. Then, you run the top of the cut-out paper with double sided tape and slide it in the envelope. This is a great way to surprise your penpal and plus, they will probably think you're more thoughtful than you actually are!