What type of underlayment is it? Where is it coming from? Posting a picture would help illustrate what you have to work with.
Underlayment like a Masonite or like an Oriented Strand Board? Or something else?
Neither one of those is going to look like ship lap, even if you run it through a table saw and cut it into “boards”.
Please send us a picture of your underlayment. Thanks
If you are looking for a thin membrane, you should be able to use the material that is put down by roofers on top of the decking before shingles are lain.
Hi Lawrence, here's a couple of videos, hope they inspire your imagination!
Get it cut into strips at the hardware store when you buy it. Then attach it to the wall with nails. I like using a nail gun because it goes so much faster. If you use glue, your walls will be ruined when you eventually take it down.
Use a jig saw to cut around outlets and other obstructions.
Check this out: http://www.rbtbags.com/2016/05/diy-shiplap-from-underlayment-worth-it.html