The lasting image of WWI seems to have been the trench warfare that dragged on for 4 years. However, the Germans seemed to have advanced rapidly toward Paris from August-September 1914. What were infantry tactics on the Western Front like prior to the Battle of the Marne (during this "opening" stage of the war where mobile warfare was still widespread)? What did these early battles (Leige, Namur, Mulhouse, Haelen, Charleroi, Mons, etc.) look like? Were there battalions of men moving to meet the enemy or was it more like strings of men spreading out and engaging in small unit tactics? Did these battles reflect the same type of combat that was used during the 1870 Franco-Prussian War or had tactics changed since then?
In essence: I don't have a clear picture of what infantry warfare looked like in the late 19th/early 20th century prior to the trenches of WWI. I took a military history course back in college, but everything was talked about at such a high level; boxes and arrows symbolizing entire armies advancing, meeting, and retreating on a map... I never knew what was actually happening on the ground during a battle. Pictures/diagrams would be great too.
The tl;dr is that in cases like Mons it was the British entrenching themselves (in admittedly low, about knee or waist high trenches) and firing upon advancing Germans. In cases like Mulhouse where it was a battle predominately fought in the suburbs it would be literal street for street door to door fighting; Mulhouse is an exceptional case in my opinion because of how much friendly fire and excessive ammunition usage there was but that's for another day. French tactics during their withdrawal is a little too varied to pin down on a specific 'doctrine' of atrophy but generally you can assume they would stagger artillery pieces whenever they could (so that whenever the Germans managed to take 1 they'd have 4 more firing on them still), use rivers to their advantage, and generally use the terrain to their advantage to bleed out the advancing Germans.
In the case of literally how the infantry organized themselves it would be akin to Franco-Prussian War tactics. Loose skirmish lines where men have 10ish feet between them about 70 men wide and would advance in 'waves'. These could break down and reform together at whim (what is traditionally called 'infantry chains') from a battalion level down to a platoon (~70-140 men) level. This gave tactical flexibility but also allowed officers to keep a handle on their men. This is a basic idea of how everyone organized their infantry and it's how the Germans would advance albeit with the addition of machine gun teams to be used as officers deemed fit; by this time however most company's would only have 1 or 2.
TL;DR of the TL;DR:
Generally infantry would be organized in skirmishing lines / infantry chains. How they would be specifically deployed is too varied to make any grand conclusion. They would fight in suburban environments, proto-trenches, open order, behind trees and rocks and brushes, or take hold of buildings and lay waste on advancing Germans. It just depended but the general organization is there. Large skirmishing lines.