My house has fiber cement siding and I want to attach a decorative sign to the siding. I'm told that I need a carbide tip drill bit to make a clean cut. But what type of screws should I use? And what length is recommended? (the house is wood framed).
Just use ordinary wood screws.
Beneath the siding should be 3/8" or 1/2" sheathing. That is what the screws will really bite into.
Just make sure your screws are long enough to go through the sign and siding and have at least 1/2 of length beyond that. If you're concerned about water penetration squirt a little bit of caulk in the hole before placing the screw, but doing nothing is probably fine too