Study details for Colonial-style architecture. Refer to similar houses in the area or in reference material. Check historical records for photos of authentic houses from the period.
Take care to plan proportion and scale of the proposed porch in relation with the house. Sketch it to scale on paper or construct a model to determine the visual effect. Adjust the proportions of prominent details, such as posts, railings and steps until they appear pleasing.
Mirror signature details, such as roofline, foundation, eaves and molding in the porch design. Repeat characteristics, like symmetry and crafting of trim, that are particular to the house.
Choose reproductions or synthetic materials that carry the architectural theme if original materials are difficult to find. Ensure that construction details are structurally sound and watertight.