How can I install shiplap on a wall that is slanted?

This is a link to faux shiplap. It's images so if you see something you like, click on the image and it will take you to the page. I hope that helps. https://www.google.com/search?q=faux+shiplap&rlz=1C1BLWB_enUS700US700&espv=2&source=lnms&tbm=isch&sa=X&ved=0ahUKEwivzMXqq5XTAhVB74MKHdlUBhkQ_AUIBygC&biw=1024&bih=690

Use both wood glue plus nails or wood screws, and use a level for each board.

You will have to use a miter saw and cut each end so that the rest of the board is level to the eye

Absolutely use a level. I would start at the top so full boards are visible on the entire wall. The smaller board would be on the bottom covered with baseboard if you choose. You could 'dry' fit the wall to see how you like it. It's a popular look and great looking wall when completed. Best of luck.