Whenever you replace an old, pre 1960 wood sash window with a vinyl replacement pocket window, there will still be an old wood sill and exterior wood stops showing. You can paint the wood to match the new white vinyl window, but paint doesn't last. You can hire someone to bend white aluminum and attach it to the wood with nails, but white aluminum doesn't match white vinyl, the nail heads are unsightly, and if you scratch aluminum, you will have to paint it to avoid rusting. A better option is to wrap your exterior wood in a vinyl product that is maintenance free, attaches with common adhesives such as silicone to eliminate exposed nails, and since the vinyl is white throughout, scratches are virtually invisible.
The pictures on the left and right show the outside wood stops being covered using the 1" X 2" angle trim. The outside face of the window was 1" from the stucco surface, and the 2" portion covered the outward facing wood surface.
If you need to trim one or both sides of the angle, you can use a plastic cutting tool like the one on the left. You just need to have a straight edge as a guide as you scrape the plastic. It only takes a couple of scrapes to cut through the plastic. You can also paint the product if you need to match a custom color, however, keep in mind that if you paint, you will have a maintenance issue if the paint scratches or fades. A can of spray paint for painting plastic patio furniture will work.
If you have a section of wood that is larger than the angle trim pieces, you can combine angle trim and flat trim to cover the post. In this picture, a piece of 2 1/2" flat trim was applied to the center face of the wood, and then a 2" X 2" angle piece was applied to both sides. The angle overlapped onto the flat piece, and the return leg went back to the window frame.