The English Comissions of Array (the closest thing to a "feudal levy"), worked like this:
The King, having calculated his manpower requirements for a campaign, issues requirements for each county and town.
Either the county sheriff or a specifically appointed Commisioner of Array breaks this down to the level of the hundreds, and how many men should be supplied from each.
The hundreds are then asked to find and equip volunteers. If there aren't enough volunteers, men might be chosen or, if the sheriff or his men, or the Commissioner or his men, are corrupt, they might choose an entirely unsuitable man for war and force them to pay a fine to get out of fighting.
The towns, meanwhile, will be negotiating with the King to send a payment or reduce the number of men required, usually offering to send a smaller number of better equipped men.
The county levies are gathered and marched to the central meeting point. Often they will be given money by their community or the Crown to pay their way to the meeting point, and will receive wages from the time they leave their county until the time they are dismissed from service.
Things are a little different if you're a farmer as wealthy as knight, but not interested in warfare. The Edwards (especially Edward I and II) frequently ordered all those with a certain amount of money, land and/or goods (usually 40 pounds) to be knighted, which meant they owed him service. Mostly this targeted the squires and men-at-arms who had put off knighting because of the financial and social burdens of knighthood, but wealthy men with no desire for war were still required to be knighted and fight.