To begin building your table saw jointer jig, you'll need a piece of 3/4" plywood measuring 12" wide by 48" in length, and a 1x6, cut to 36-inches in length. In a few steps, you'll also need a 60-inch long strip of stock ripped to dimensions of 3/4" x 3/8", but you can use pretty much any straight piece of scrap stock for this piece.
Set up your fixed-base router with a 1/2" straight-cutting bit, and place it into your router table. Adjust the height of the bit to about 1/8" above the table surface.
We're going to be making two slots in the 1x6, each about an inch in from the ends of the board (perpendicular to the board's long axis). Each slot will stop 3/4" from the edge of the board.
Set your router table's fence 1" away from the edge of the bit. Next, make a pencil mark on the fence 3/4" in each direction from the edges of the bit. These marks will denote the start and stop points of the slot.
Start up the router and ease the board down onto the bit with the leading edge at the far pencil mark. Then, guide the board along the fence until the trailing edge of the board has met the trailing pencil mark. Lift the board off the bit, and repeat for the slot on the other side. At this point, you can flip the board over and make the first slot cuts on the opposite side of the board in the same manner.
Raise the router bit another 1/8" and repeat the four slot cuts, using the same procedure. Continue this process until both slots are cut completely through the board.
SAFETY TIP: Be very cautious when pushing end grain against your router table's fence, as end grain can bind against the fence, which can lead to kickbacks. Work slowly and methodically, and consider dropping your router bit speed down a little bit to prevent bit burning.