Writing in a straight line on opaque paper (where a lightbox and guidelines beneath the paper won't work), and where drawing pencil lines and erasing them is too time consuming or impossible to erase without disturbing the paper (or they can't be seen, as on some metallic papers), this method works for me.
Using an envelope as an example,
1. place envelope atop a guideline sheet, aligning the lower edge with any straight line such that you can see guidelines on both the left and right side of the envelope. (If the piece is so large as to cover the guidelines completely, tape two sheets together side by side to create a larger guideline sheet)
2. place the laser level such that it follows a guideline across the envelope. In this example, I'm aligning it with an x-height line.
3. Write your first line along the laser light. (By lucky coincidence this particular level shoots a beam 1/16th wide, so I confine my miniscules to the width of the beam if I am writing this size).
4. Move your work up up one guideline, keeping the guideline sheet and laser positioned as originally. This keeps you writing in the same position. Write your second line. Move your work up a line, write your third line, and so on until finished.
5. You're done, or ready for a new envelope, with no marks on the finished envelope.
There is a device called a SliderWriter which does a similar thing, but is more expensive ($60 on Amazon) vs. about $15 for the level. You're also limited in the size of your work with the SlideWriter--about the size of an 8.5 x 11 inch sheet, whereas with the level, any size is possible, even wall size or a wall itself. Also, the Slidewriter has a big footprint on your desk--the level is small, and can be positioned any distance from your work, as long as you don't block its light.
I'll often use the level without the guidelines at all, just making sure the light is parallel with the bottom of the envelope, but if you want exactly equal spacing between the lines, use the guidesheets.