Measure the space between the supportive posts around the deck and the space between the deck and the ground.
Cut 2-by-4s to fit between the support posts, adding extra to allow you to nail the 2-by-4s into the support posts. For example, use 40-inch 2-by-4s if the support posts are 4 inches wide and 36 inches apart. That would allow you to utilize 2 inches to secure the 2-by-4s on each post and leave space for the 2-by-4 that will connect that post to the next.
Nail a 2-by-4 to the inside of two support posts along the top, right under the deck. Secure another 2-by-4 to the bottom of the support posts. The result should look like a rectangle. You'll have to crawl under the space to attach the 2-by-4s to the inside of the posts so they aren't visible when you're done. Repeat this process until each support post is connected to the next with a 2-by-4.
Trim your skirting material to fit the measurement between the deck and the ground. For example, if the top of your deck is 48 inches from the ground, your skirting material should be 48 inches tall. Common skirting material includes lattice, beadboard, vinyl siding or wooden planks that match the deck wood.
Position the skirting material so the top is flush with the deck and the bottom brushes the ground. Use a nail gun or hammer nails through the skirting material into the 2-by-4s along the top and bottom. Repeat this process until the entire deck is outfitted with the skirting material.
Trim any excess skirting material with a saw if necessary. Sand and paint the skirting material if applicable.