If necessary, remove a few pieces of siding, nail the undersill, then replace the siding. Tack Z-flashing over the channel If you need two or more pieces of flashing, overlap the joints by 4 in ...

If the siding is vinyl, choose a primer such as Behr’s acrylic interior/exterior multi-surface stain-blocking primer and sealer ($15.98 at Home Depot), which is labeled for use on vinyl, as well ...