For example how would they gather the food, armor, weapons, ammunition, etc. how long would it take to gather these materials? Who get paid for the materials gathered, I'm assuming of course that the people who supply these materials would want some compensation for them. I know that camp followers were a thing with pre-modern armies, how would they factor in to all of this if at all?
I posed this question last weekend, It got a few votes but no answer. So I'm posting it again to see if I get better luck this time.
In Western Europe there was a very specific "Campaigning Season", starting roughly in March (the "Month of Mars", the ancient roman god of war), and ending around October, when the peasants had to return to the fields to harvest their crops.
Consequentially, most preparations for war would take place in the months between October and March.
Actual preparation time depended from case to case, but was most importantly influenced by the size of the army. The only specific amounts of time I know are purely anecdotal. For example, in 1154, Emperor Frederick Barbarossa collected a small army in Tirol (consisting only of professional soldiery, thus eliminating any constraints poised by the harvesting season) and set off for Italy in October. The Italian Cities, who had been ignoring Imperial authority for the better part of a few hundred years, could not raise an army to oppose him in the winter. Barbarossa laid siege to the cities that did not recognize his authority, and the last to capitulate before he set off for Rome after Easter was Tortona, in April 1155. Thus two months were not enough for the Milanese Capitains to rally enough men to oppose the Emperor and rescue their allies in Tortona.
However, in Italian warfare, fighting between states usually entailed relatively small armies that could be raised in a much smaller timespan.
While not specifically answering your question, you might want to give /u/backgrinder 's interesting response on the logistics of supplying an army of English longbowmen a read.
Only real knowledge I have is of post Conquest England up till end of hundred years war. One of the big issues was that most of the men required for a campaign came from the nobility calling in there dependents so English Kings had to rely on their nobility to muster the required man power. Good example of how this hampered military campaigns is Edward II's attempt to defeat the Scots at Bannockburn with one of the key english nobles sending barley any troops. I'd recomend Juliet Barkers book 'Conquest' and 'Agincourt' as it goes into details of how English troops were raised for the Hundred Years war. Hope this has been helpfull.
Sorry for the late response, I was at work all day. Thank you guys for the responses. If I could trouble you for some books or articles for more information I would appreciate it. I'm actually looking to DM a game set in a semi medieval world and I want to make the story as detailed as possible. If I could also pose a follow up, Would armies on the march have all the food they think they would need with them from the start or do they feed off the land as it where? I'm going to guess that the answer is that is that it depends but I want to know what you guys have to say.