1. Cut the new and old parts at the cut-and-join location, and bevel the parts.
2. To cut-and-join the new part, cut at the locations indicated in the figure below and bevel the cut-and-join locations of the new parts.
3. When installing the new parts, trial-fit them to the body, and position each part so that the each section alignment matches the body dimensions.
4. After temporarily installing new parts, make sure the related parts fit properly.