Wolverine Siding and Vinyl Carpentry Soffit and Decorative Trim - Manual (Page 45)

Measure the bottom of the opening and add 7" (3-1/2" extra
for each side) if the side lineals are also 3-1/2". Add 10" total if
using 5" lineals.
Trim 3/4" tabs from the lineal as shown.
Snap the lineal in place and secure by nailing (or stapling) it
through center of the nail slots.
44
Wolverine Fit and Finish Installation Guide
finished
assembly
with side
lineal
window
frame
view of
notching
from back
side (same on
both ends)
3-1/2"
3/4"
3/4"