How do I do shiplap with an uneven wall?

Thank you now I have a better idea.

Great! Good luck and keep the string the same distance from the wall. You could use a Combination square to set and check the distance. I’d do it every couple feet up the wall to the ceiling You could use those removable hangers as they are molded plastic and should be the same distance and use mason’s string A long level at least 4’ long will help, 6’ would be better but more expensive It will also be helpful in locating dips if the dip isn’t longer than the level. Anything shorter than 4’ will not be very accurate. You can use a pencil to outline the area and then feather the mud to the line.