Extending the jambs, applying the base casing-then the applied mantle mold and dealing with the now level window sills perched utop the unlevel original sill one pc at a time?
Be reminded of preassembly at a Katz Roadshow clinic my lumberyard put on right about this time.....
Plan and cut the jamb extensions for the windows-remember these are Andersen replacements-they are packed out to the ext. sash stop. How that ran with the now newly veneered drywalled walls in a 145 y.o. house was anything from the same or even. So precut those extensions, fit and shape the stools. Preassemble using glue and both Kreg and regular screwing.
Next, fit and apply the 1x4-1/2full flat stock.
Apply the mantle mold to the face of the flatstock-doing your best to register the edge.