"On the 6th," James "Pete" Longstreet remembered years after the war, "[My regiments] were marched out, formed as a brigade, and put through the first lessons in the evolution of the line, and from that day to McDowell's advance had other opportunities to learn more of the drill and of each other."
The new brigadier general had been at Manassas for less than a week, but he was already giving them more training than they had received yet. In mid-1861 both Northern and Southern armies there was a not a dearth of fighting men, since many of the first volunteers had seen some action or training in Mexico, but there was a complete lack of experience in working together as a unit. Most militia were between 60 and 120 men - a company - which was a suitable size for fighting Indians or putting down slave revolts (the two biggest threats in American national security, 1830 - 1860) or even putting on a good parade, but would not be enough to fight even on the small scale that most policy makers imagined in 1861.
The North had an advantage in that it still possessed regular army units - men experienced in fighting as a unit. But Brevet Lieutenant General Winfield Scott had decided regular army units would not be split up for the war (in fact, most would stay on the frontier fighting Indians, what they were most familiar with). In practical terms, this meant the North was not much more ready than the South. They both had a critical shortage of non-commissioned officers (sergeants and corporals) who could keep new recruits in line (the phrase "to keep in line" refers precisely to this - sergeants forcing privates to stand in the battle line during drills or combat).
To complicate matters, the companies that had been local militia and the companies of greenhorns all had different levels of training, and when eight to ten of them were stuck together in a regiment it became unwieldy. When three or four of these regiments had to work together, each with a colonel who often had drastically different levels of military experience and had put his regiment through drastically different levels of drilling already, it was a recipe for catastrophe. So, it was a top priority for the professional infantryman Longstreet to get his Fourth Brigade a crash course in battle formations - though Longstreet himself had never commanded a unit larger than a company.
On July 8, the matter took on greater urgency. Whether the commanding general of the Army of the Potomac (the Fourth Brigade's army) G.T. Beauregard had direct confirmation or not, it was certainly clear from the multitude of pro-Southern informers in and around Alexandria that the Northern men under Irvin McDowell were gearing up for something. So he issued an update to his previous order of defense to give each brigade a particular mission for when McDowell advanced.
Beauregard had decided to use Bull Run and the Occoquan River as his walls. Bull Run is a windy river with steep banks, which makes it hard to cross, but the water is rarely so deep that were the banks are lower it can't be easily crossed. The junction of the Orange & Alexandria Railroad and the Manassas Gap Railroad (in present day Olde Towne Manassas) was the area's prize, since Northern control would keep rail traffic from taking the direct line from Front Royal to Fredericksburg, making both cities that much harder to defend. So Beauregard rightly assumed McDowell's plan would involve moving to Centreville and traveling one or both of the Centreville Road and Union Mills Road to the railroad junction (today, VA 28 and an extension to Rte 659 no longer in existence that runs through Hemlock Overlook).
View Defending Manassas Junction in a larger map
At this point, Bull Run forms a large bend and here Beauregard had his men begin digging their trenches (contrary to popular belief, fortified positions were constructed from the very beginning of the war -- just not enough of them), with one brigade assigned to guard each ford in the event of a battle. The Second Brigade, under the highly regarded Dick Ewell, guarded the ford at the "right" of the Confederate line (as determined from the perspective of the commanding general, standing behind his army and facing the enemy, in this case at Manassas) called Union Mills Ford. Philip St. George Cocke's large Fifth Brigade was responsible for guarding the left at Ball's Ford. Just to keep an eye on McDowell, Beauregard put a battalion of Louisianian rough-necks who had taken to calling themselves the "Tigers" and some artillery and cavalry under Colonel Nathan "Shanks" Evans at the Stone Bridge along the Warrenton Turnpike (US 29).
"Neighbor" Jones and the Third Brigade were to guard McLean's Ford, and the former-South Carolinian Congressman Milledge Bonham would put his headquarters in Fairfax Court-House (City of Fairfax), with his First Brigade spread out as far as Vienna, to provide early warning for when McDowell was moving. Bonham would fall back down Centreville Road (VA 28) fighting when the Northerners came and then take up position on the southern side of Mitchell's Ford (where Old Centreville Road crosses Bull Run today). To guard the last ford and to make sure Bonham could make it down Centreville Road and back into his defenses at Mitchell's Ford, Beauregard stuck Longstreet at Blackburn's Ford, the nothernmost tip of the outward most tip of the bend (where present-day VA 28 crosses), with Jubal Early's Sixth Brigade nearby to go wherever it was needed (most likely across the river to help Bonham).
"On the north bank," Longstreet remembered later about his position, "Stood a bluff of fifteen feet overhanging the south side and ascending towards the heights of Centreville." In other words, his men would be almost underneath the Northerners if they stayed south of the river. So he moved them north of it to build his defenses with the water at their backs. The men of his brigade were "more familiar with the amenities of city live than with the axe, pick. spade, or shovel. They managed, however, to bring down as many as half a dozen spreading second-growth pines in the course of two days' work, when General Beauregard concluded that the advanced position of the brigade would mar his general plan..."
So the Fourth Brigade went back to the south side, under the bluff, "a position," Longstreet noted dryly, "only approvable as temporary under accepted rules of warfare." It was perhaps the first moment of Longstreet's disapproval of Beauregard, which would grow much greater over the course of the years. Still, even near the height of the enmity between the two, when he published his autobiography, Longstreet concedes that the position "proved a favorable exception between the raw forces of the contending armies."
Now he had to build new defenses, drill his men, and wait.