What is the cheapest and easiest way to shiplap walls?

Hope: maybe these will help

https://www.maisondepax.com/how-to-plank-a-wall-diy-shiplap/

https://hoosierhomemade.com/diy-shiplap-wall-for-under-40/

Here's an easy and cheap tutorial for faux shiplap, which might work for your needs: https://www.youtube.com/watch?v=5hq9vfjCNXU

This was the easiest but probably not the cheapest: https://www.craftylittlegnome.com/faux-shiplap-peel-and-stick-wood-plank-wall/

You could try this: https://www.hometalk.com/diy/build/walls/gorgeous-shiplap-at-a-fraction-of-the-price-22724489

One of the cheapest ways to shiplap a wall is to use printed plywood. You can cut each 6’ panel into as many thin planks as you wish. Have the planks slightly overlap at the upward sides. Or fake it with wallpaper in a ship lap design. Hope, there are so many options available today, many of them actually are shiplap, and others are representative of shiplap. Give yourself plenty of shopping time, then try one. Jewell

Here's more ideas from Hometalkers

https://www.hometalk.com/search/posts?filter=shiplap

I have not personally tried this, but I have heard of others using permanent marker to "draw" it onto the wall.

You can actually purchased unique, pre-made shiplap that is so easy to install! https://bachelorettepadflip.com/2019/04/30/updating-boring-walls-with-woody-walls/

Get the look with paint! Here's how I did it: https://thehoneycombhome.com/shiplap-accent-wall-the-easy-way/