How do I do a shiplap wall with a Sharpie?

This should help https://www.hometalk.com/diy/decorate/walls/diy-shiplap-wall-done-with-sharpie-43429591

They just drew on horizontal lines. Be careful if you mess up or get tired of it Sharpie doesn't take to repainting very well. Must sand and cost with stain blocker.

Here's another great tutorial on creating a shiplap look with a sharpie

https://www.hometalk.com/diy/decorate/walls/4-shiplap-wall-with-sharpie-43460491

If you're trying to do a faux ship lap effect with the sharpie you'll want to make sure all your lines are straight and the same distance apart. I'd start with a good laser level that can project the line onto the wall. Then using a 4' straight edge hold it up to the line tightly and draw your line. Overlap a little and do the next section, and so on and so on until the room is done.