When installing or replacing siding I always flash siding joints by using tar-paper splines at all of the vertical seams.
For clapboards make sure the tar-paper spline overlaps the clapboard course below it. This overlapping coverage will ensure that moisture drains out and away from the house.
Flashing Siding Joints
The vertical seam at the window also receives a spline. windows, doors and corner boards all create a continuous vertical seam that can allow moisture to enter. Flashing these areas is as critical as it is to install proper fitting clapboards.
I make sure to overlap this tar-paper spline on top of the clapboard that is installed at the bottom of the window. [not shown]