To begin building these barn doors, the first thing to do is to measure the door opening, and determine if you'll need to build a door jamb. If the raw framed opening is sufficient (since this is for a barn or a shed), you likely won't need to build a door jamb.
Also, for this set of doors, we're making an assumption that the building has a raised concrete floor (or at least raised piers along the perimeter) that extend a few inches above the ground. This is common in barns, and it will allow us to have the bottom portion of the door flush with the outside of the bottom concrete threshold of the door opening.
The doors as noted in these free woodworking plans are for a 4070 door opening (4-foot wide by 7-foot tall). You should allow a quarter inch on each side of the opening for the door reveal, so adjust these plans accordingly. Also, be sure to cross-measure the opening diagonally to check for square. If your opening is not square, you may need to adjust your door to accommodate the opening.
Once you know what adjustments you'll need to make to the free woodworking plans you downloaded in step 1, the first step is to rip a 45-degree bevel along one edge of an 8-foot long 2x6. This beveled board will be cut to width, and will form the top rail of the bottom door and the bottom rail of the top door. Adding the bevel will keep wind from coming through the gap between the dutch doors.
After ripping the 2x6, cut it to length. If your opening is 48-inches, cut it to 47-1/2" in length. Set the remainder of the beveled board aside until later.
Next, cut another 2x6 to 47-1/2" in length, plus two lengths of 37-inches for the stiles.
Note: In the upcoming steps, we'll focus on building the bottom half of the door first, as the top half will repeat the steps, only with minor adjustments in sizing.