I followed
these plans, more or less.
Get about an 8 inch scrap of 4x4 post. Drill a 1 inch diameter hole into the end grain and about 6 inches or so deep. then drill 0.5 inch diameter holes in from each of the 4 sides at a 45 degree angle so they all meet at the bottom of the 6 inch hole you already drilled. I got an empty peanut butter jar because I didn't want any broken glass in case the thing fell, and drilled a 1 inch hole in the cap and screwed the cap to the block of wood in line with the 1 inch diameter hole. Attache the jar to the lid and hang the whole thing under an overhang of some sort, near the wood damage.
The bees come up to the block, see the 0.5 inch diameter hole just begging them to investigate. They go in and crawl uphill at the 45 degree angle. Everything is dark until they get to the top, and then there is the large 1 inch shaft that drops down into the jar and there is lots of light down there, so they follow the light out. Except then they are in the jar.
Here's a nice site too.
And I made my trap a little more tricky by cutting off the top of a water bottle and inverting it like a funnel and attaching that to my peanut butter jar lid so the bees would have an even harder time finding their way out. Kind of like this.
The life cycle of the carpenter bee is that they are active in the spring and lay lots of eggs, and then those eggs hatch in the late summer, and they are active again. So it's a bit of a lull here now and I haven't caught anything. But when they hatch, I'll get them.