This is the way I see it: All the elemental jutsus - e.g (earth wall, fire ball, water attacks, different types of clones) all stem from the original creation of things jutsu. The So6P was able to weave and transform chakra into real tangible objects of any element. Seals are a primitive, means of emulating the sage's ability - they guide the flow of chakra to form objects/attacks in the same way that letters combine to form words. I think the direct disciples of the sage must have either learned the foundation of seals from him, or invented them as a solution to emulate his abilities.
Most people in the Naruto universe are like Rock Lee, absolutely useless at utilizing their chakra, but I'm willing to bet that even he could perform some very very crude jutsu if he practiced the seals a lot - as everyone has chakra. Others are gifted with a genetic predisposition to specialize in transforming chakra into specific elements.
Lets say the seals for the earth wall go like this:
Handsign 1: Charge up chakra in hands and feet
2: Release it to target location
3: Let it orientate itself to rise up
4: Guide it to critical mass -- (preparing it for rapid transformation into earth element material)
5: Release! (Hands to the ground) -- Chakra rapidly transforms into earth elemental as it rises from under the ground and pushes up a wall.
Most of the fodder ninjas probably know how to do step 1-4, from practicing their own elemental type jutsu. The limiting factor would be step 5, where they'd probably only get a conversion rate of lets say 20% to earth material, as opposed to lets say 95% for talented and well practiced earth users. Given the fact that Ino was guiding them via her mind, groups of ninja would co-ordinate all target the same spots, creating walls from the efforts of dozens of ninja each, as opposed to what would otherwise be pretty small, pathetic walls.