Purchase 4-by-4 posts that reach from your existing porch railing up to the ceiling above the porch area. If you don't have a railing, the posts will need to measure from the deck to the ceiling portion of the porch. Trim posts with a reciprocal saw.
Install base strips to the outer surface of the 4-by-4 posts using a drill and wood screws. You can purchase the base strips at the hardware store, and they allow you to easily install screening material without the use of nails or staples. For 4-by-4 posts, use base strips that are 3 ½ inches wide. Trim strips if necessary with tin snips.
Stretch screening material across two posts and secure to the base strips using a screening roller. The roller will push the screen into the appropriate grooves on the base strip to keep it secure. Trim away any excess screening material with a utility knife. Work your way around the entire porch.
Place a cap trim piece over the base strip and tap with a wood or rubber mallet to secure. The cap trim covers up the base strips and edges where you installed the screening material. Trim the cap strip with tin snips if necessary.
Install a screen door at the porch opening. Install the hinges to the wooden posts to attach the door, and place the latch on the opposite wooden post.