While the myth of veteran soldiers throwing their knapsacks away 'en masse' after the first battles of 1861 - 1862 is, in fact, a myth - it is also true that large numbers of soldiers did use bedrolls.
Sometimes bedrolls were temporary expedients to replace lost knapsacks (veterans soon learned to ignore the command to 'ground knapsacks' - even if you won, you still might never see your gear again). And some times they were preferred by the soldier over the knapsack.
And, it at least one case, it was done deliberately. The 9th NYVI (Hawkin's Zouaves) was ordered to leave their knapsacks in Washington when they marched to Antietam in 1862. They weren't happy.
For the campaigning reenactor, the bedroll is an option to the knapsack (and sometimes - as it was then - it is in addition to the knapsack).
Lets explore this simple technique.
First, you need a blanket. While you roll might contain your blanket, gum blanket or poncho, and a shelter tent - the most important part is the blanket.
There are few really good reproduction blankets available. The reproduction Weissert blanket, formerly sold by the Wisconsin Veteran's Museum is an exact replica of a almost unused 1864 US blanket. At $125, it is pricey - and they are out - so you have to find a used one. They will have a CS blanket out in 1999. Country Cloth has had good, but often out-of-stock blankets. The repro US 1861/1862 'emergency' blanket sold by Fall Creek isn't too horribly bad (the ends need to be un-hemmed).
However, both the US and CS armies used a lot of non-descript blankets. For a stop-gap (until you find a good blanket), you could get a good, 100% wool, 5 pound blanket that is at least 64" x 72" and rework the hems if necessary. Avoid any non-wool content (you can see polyester 20 yards away). Avoid the awful modern US government pressed foam and trash disaster blankets. Avoid modern US military issue blankets (polyester!). Avoid US Army blankets - even the WW2 era 'mustard' ones. Good colors are gray, blue, white and tan. Avoid any blankets with modern markings on them (like East German Army blankets that are marked "NVA").
Lay the blanket out and fold it once on the long axis. Lay out your essentials (extra drawers, extra socks, extra shirt, toothbrush, &c) on the blanket. The haversack is for your rations. Now - lets finish it up.
How did 'the boys of 1861-65' do it? In his fantastic memoir The Story of a Common Soldier, Leander Stillwell says: "So we would fold in our blanket an extra shirt, along with a few other light articles. Roll the blanket tight, double it over and tie the ends together, then throw the blanket over one shoulder, with the tied ends under the opposite arm."
John Worsham, in his One of Jackson's Foot Cavalry, gives another view. "I had a very good oilcloth haversack to carry my rations in, a tin cup, a splendid rubber cloth, a blanket, a pair of jeans drawers [!], and a pair of woolen socks;. . .The socks and the drawers were placed on the blanket, the blanket was rolled up with the rubber cloth on the outside, the ends drawn together and fastened with a short strap. To carry this, we put it over our head and let it hang from the shoulder."
If your poncho is rolled on your blanket roll it will keep it dry, but is unavailable when it rains.
Likewise, the inclusion of the shelter half requires that you have a historically accurate reproduction shelter half rather than a heavy modern canvas shelter tent, which won't roll decently.
The roll needs to be slightly loose, rather than rolled tightly. It also needs to be 'twisted.' It will be more comfortable to carry.
Another option is to make the shorter 'hobo roll' and tie it off with a 'U' shaped length of hemp, sisal or cotton rope on both ends, using the rope 'U' to sling the roll over your back. Some 'skinners' sell blanket roll strap sets for this. We wonder where Johnny or Billy would have gotten these - as he never saw anything like a reenactment "skinner's row" during his years of service. They do appear, however, in an early war image of a private in the 2nd Rhode Island Detached Militia - then-Colonel Burnside's regiment - which impressed 1861 Washington with their 'business-like' air and appearance in their overshirts, jeans trousers, and blanket rolls prior to 1st Manassas.
This brings us to the image below. This is Private William A. Wyatt, 117th Regiment, Illinois Volunteer Infantry. Despite the poor quality of this reproduction of his image, you can notice the way his blanket roll is twisted. Also notice how high he wears his haversack and that it is not underneath his waist belt - which is worn high itself, around the belly button and not down on the hips!
Private Wyatt is a fine example of the way the veteran soldiers of both armies looked. No matter if you choose to use a blanket roll or a knapsack, you need to study images of veteran soldiers, especially in the field, and pattern your look after them. Hopefully, this article has helped.