Review on Enhance Your Envelope Addressing with Lettering Envelope Addressing Stencil - Perfectly Straight Guide for All Envelopes - Ideal for Sending Cards (1 Pack) by Carrie Spencer

I used to use a ruler and pencil to mark my lines and then erase them after I wrote the address, but this caused smudges or lightened the envelope. For me, this is a great solution, although making sure you don't move it while typing takes a bit of getting used to. The only downside is that you are somewhat limited in the size you can create for your lettering. Fantastic tool if you still send a lot of "snail mail".