This information is from the The Civil War Reenactors website.

Pull the cuffs down off the sleeves. Lay the coat out, back down, with the cape spread out at the top like a fan. Be sure the seam of the cape is straight up along the line of the back seam of the coat. Pull the center vent together at the bottom. (Fig 1)

Fold one side of the coat over the center seam all the way to the opposite side seam (1/3 of the total width of the opened coat). Fold the cape 1/3 of its entire width.

Fold the sleeve across the top to the opposite shoulder, and double back any overhang. (Fig. 2)

Fold the other side of the coat the same way. The coat is now folded in thirds lengthwise, the width of the shoulders (also the< width of the knapsack). (Fig. 3)

Go to the bottom, and fold the outer edges of the skirts inward, starting from the waist or above, and down to the hem. The width of the skirt is now the width of the shoulders. Likewise, fold the corners of the cape inward to the width of this shoulders. Now the entire garment is one long rectangle the width of the shoulders. (Fig. 4)

Fold the cape downward over the collar, and the skirt upward to the bottom the cape. If your coat is long, it may have to be folded higher. At this point the coat can be rolled if necessary for cavalry or for garrison display. (Fig. 5)

For packing in the knapsack, fold the coat again at the middle. The coat should now be the same depth, as well as the same width, as the folded knapsack. (Fig. 6)

You may need some practice to get the overcoat folded correctly. Large coats need to be folded closer. The overcoat should not hang out so far that it droops on either side. If it is loose, you will lose it on the march.