Today you can obviously see who has the 'best' army by just looking at the advanced training and equipment certain countries use. 500+ years ago you can see who has the best army by who has the best trained, best fighters, more nobility in giant ass armour etc.
But once gunpowder was invented and in WWI what did it mean to have a good army? You can have good generals and that makes sense but an army itself seems something anyone can do.
Walk at the enemy shoot them and die. You can't exactly be good at that.
As well a lot of people say that at the time of WWI Germany had the greatest army in the world...but that's the same thing there.
I understand my thinking about what involved wars and battles back then is incorrect so where am I incorrect and what makes a good army then?
Having a good army, from about 1870 onwards meant having one that was:
A) Large; possessing a standing, regular army in the 100 000s, with a large reserve and national guard (Territorial Army, Landwehr), and a substantial part of the male population having been conscripted for a year at least of active service.
B) a good system of roads and railways, ESPECIALLY railways, to easily and swiftly mobilize your forces.
C) an army that was well equipped, i.e practical uniforms, modern weapons and equipment (bolt-action rifles, quick-firing breech-loading artillery), and one that, by the early 20th century, preferably made use of or had access to, telephones, telegraph and radio.
D) an army that was well-trained, carried out fairly regular exercises, and sought to incorporate infantry, cavalry, artillery and all other branches into an effective system on the battlefield.
E) a professional, well trained officer corps, with access to officer schools and academies, a large corpus of national and international literature and journals to consult, opportunities for positions as observers/attaches in foreign countries and conflicts, and a well organized war ministry and general staff.
F) an effective government, a strong economy, and in general access to considerable funding to keep your armies up to date.
Countries like Germany, France and to a certain extent Britain managed to do this quite well; Russia was doing well from 1913-14, Austria-Hungary and the Ottomans, much less so!
Furthermore, your assertions that 'a general' or 'an army' is something 'anyone can do', and that you just 'walk at people, shoot them and die' are pretty far off the mark. You needed to have a fair amount of income, good social connections, and a LOT of education to be a general, and it was damned hard work even in PEACETIME, let alone at war. The German Army, hell virtually every other European army, had MASSIVE bureaucracies and staffs just do deal with all the organising and paperwork, and even more to deal with logistics, supply, and technical jobs.
Moreover, tactics were changed greatly by the 1870-71 Franco-Prussian War; the days of line formations and volley fired muskets were well since over in Europe (/u/Elos has made extensive posts about this).
Further reading:
Geoffrey Wawro, "Mad Catastrophe"; Prit Buttar, "Collision of Empire's"; Margaret MacMillan, "The War That Ended Peace"; Peter Hart, "The Great War: A Combat History of the 1st World War"; Frank Davies & Graham Maddocks, "Bloody Red Tabs: General Officer Casualties of the Great War, 1914-1918"; Gordon Corrigan, "Mud, Blood and Poppycock"; Richard Holmes, "Tommy"; David Stevenson, "1914-1918"; Max Hastings, "Catastrophe"
Hope that helps!