Milled clapboard is 1/16" thick. Unless it's something delicate, I just use a regular utility knife with a good sharp blade (draws blood in 1st pass) generally free hand for doors and windows openings, but steel ruler for large cuts on 1st pass. Delicate stuff - #23 scalpel - fresh blade. Several passes regardless of cutting tool.
Now anybody got a source for milled clapboard that doesn't require involvement of a loan officer?