Determine how many inches of space you need between the balusters. The common spacing is 4", which is code for many cities. Measure the length of all the railings to figure out how many balusters you will need.
Figure out the height of the baluster based on the height of the post. Cut the desired number of pieces that will be needed for the railings to that exact height with a table saw. These will be pieces of pressure treated 2x4. Measure the length between each post and cut the 2x4 to the correct size based on these measurements. Repeat for the number of balusters you plan to use. These will be used for the top and bottom rails.
Find the center of the first railing and mark with a pencil. The first baluster will be installed directly under this pencil mark and this will be the beginning point of the installation. Continue down each side of the railing, making a pencil mark every four inches. Repeat this process for each railing until you reach the end posts of your deck.
Take a baluster and line it up with the pencil mark beginning on the far side of the railing. Drill a screw into the 2x4 and then into the baluster. This will be the top rail. Flip over the rail section and do the exact same thing with the bottom section. This will be the bottom rail.
Repeat this process for the entire length of each railing, one at a time. Firmly grip each baluster once attached to make sure it is secured properly. If it wiggles or moves, insert another screw into the rail and baluster for a stronger, more stable railing.