Slope windowsills with a piece of siding
Even if you’re doing your best to follow best practices for proper waterproofing, water can still find its way onto the wrong side of a window. It’s a good idea to slope the sill in the rough opening to help unwanted water find its way back out.
You could slope the sill itself, which requires cutting the cripples at a slight angle and causes the sill to protrude a bit on each side. Instead, rip down and install a tapered piece of lap siding. Make sure to frame the rough openings a little larger to account for the width of the siding.
And of course, cover it with flexible flashing before installing the window.