I know you asked about central Europe, but I thought it might be interesting to know how it worked in Sweden as well.

In Sweden, you would know. By old tradition, the King required the support of the Swedish estates parliament to go to war. The Swedish estates parliament was somewhat unique in Europe that it included the peasants as an estate with (at least in theory) equal power to the other estates - the clergy, the burghers and the nobility. The peasants estate was represented by self-owning/free-holding peasants who owned the land they lived and worked on, themselves about half of the peasant population (the rest being tenants of the nobility, the church or the crown) and owning around 40% of the arable soil of the country at this time.

In general, the King had little problems convincing the other three estates of going to war in Germany - the nobility saw opportunities for military careers, plunder and advancement, the priests were aboard on the idea of defending the protestant faith and the burghers were dominated by Dutch and Low German protestants who saw their interests and friends and relatives threatened by Habsburg and Imperial power, catholicism and centralisation. But the peasant estate had in general nothing to gain from war, and would provide a majority of the money and most of the men spend in the war and were in general conservative when it came to foreign adventures.

To convince the peasant estate of the necessity of war, the King and the royal council engaged in one of the first European PR blitzes. Teh campaign of propagande was aimed both at internal and external targets. And some of it aimed at external targets were intended to create an atmosphere where foreigners aided in convincing the Sweidsh peasants that war was the only proper option.

So the ideas of the Lion of the North coming to defend liberty, the true (protestant) faith and saving (northern) Europe from the tyranny of Imperial encroachment on the old rights of the Princes of the Empire were peddled far and wide by prints in both text and images, songs and poems to the extent where northern German representatives would come to Sweden to ask for aid in carefully planned and orchestrated events that would help sway the members of the estates parliament for the case of war.

Internally, the successes of Wallenstein against the north German Princes and forces of the true (protestant) faith, especially teh victory against Denmark in 1628 and the naming Wallenstein as "Admiral of the North and Baltic Sea" was used as an argument that the Imperials would not be content with enforcing centralised Imperial power in Germany, rather they would not only enforce catholic faith in the Empire, but Wallenstein was to be tasked with building an Imperial fleet (perhaps in cooperation with the Spanish fleet) in the North Sea and the Baltic Sea and invade Denmark and Sweden to end protestantism in Europe once and for all. The unity of the papists (catholics) was often brought up as a threat, and that the protestants needed to unite, or fall separately (Americans will know this reasoning from the revolutionary war), with Spain and the Empire being ruled by the same catholic dynasty and Poland-Lithania, the other enemy of Sweden in this era, also being ruled by fervent catholics.

The King and his goverment also used the celebrations around the 100 year anniversary of the end of the Kalmar Union, with plays, celebrations, prayers, generous gifts for the poor and so on to remind the peasants of the role the Royal House of Vasa had in liberating the country from foreign opression and how the King had the best for Sweden and its people in mind.

Prayer days and religious events were used to further stress how dire the situation was for the "brothers and sisters of the faith" in Germany, with prayers for the poor people being ravaged by the monstrous catholic mercenaries under the even more monstrous Wallenstein.

In the end, the peasants estate voted for war, with the argument "better we tie our horses at their fences, than they at ours" - convinced that war was coming, they would rather have it in Germany than at home.

As for how the peasants knew about the war, who was fighting and why, they had already gotten quite some from the royal PR blitz and would get further throughout the war through their local magistrate and their local parish priest.

All were by the church law required to attent every Sunday (or every 4th Sunday, or sending just one of your household if one were more than half a day's walk from church), and the priest would after the normal service read announcements and news from the royal administration, which included prayers of thanks for victories in the war, news of offensives, the need for all to do their duty when the situation was dire (such as after the defeat at Nördlingen 1634) and so on. From the early 1620s, the priests were required to keep a census that included all males above the age of 15. This census would be used to enact conscription - not by picking out men, but by picking out numbers. The old things had been replaced by the estates parliament by the early 1500s, but on the local level, they evolved into parish councils, in which peasants elected representatives to handle local legal issues (12 peasants and the local magistrate made up the local court), church issues and matters of tax, collectiv responsibilities and so on. In general, the local magistrate would conscript a number of men for the army, but generally leave the selection of men to the local parish council. A normal number was 1 out of 10 for each crown tenant or free holding peasant between 15 and 40, or 1 out of 20 for each noble tenant between 15 and 40.

Since the self-owning peasants usually could control who was selected as long as they provided the correct number of able-bodied men, they could and often would protect themselves and their families from conscription. The conscripted men were often crofters, farm hands, day labourers or even "rootless travellers" the peasants wanted to get rid of. It was not uncommon to get volunteers, by coercion, monetary compensation, promises or other incentives. Nine of the ten could all provide some money for the tenth to accept, or promise to care for his family while he was away and in the event of his death.

So, to summarize. If you were a Swedish peasant, you would know who was fighting and probably have a simplified understanding of why (to protect Sweden, to protect protestantism, the finer points of Imperial centralisation and the rights of Princes in the Empire and so on would probably not be known to you). You would know the basic combattants (Sweden, France, Spain, the Emperor, some of the more important protestant and catholic Princes) and the general flow of warfare. You would get these news from the pulpit and the local magistrate though announcements and information. Some might come from veterans that return (often wounded or maimed). You would probably personally resent the cost in lives and taxes and labour dues to your communuty (for example the work you have to do on nitrate barns to provide the army with gunpowder) and fear conscription, but if you are one of the self-owning and more prominent among the local peasants, your position on the parish council and local court gives you the ability to avoid conscription, at least. If you are successful and have money, you can always use that to give someone else in your group the incentive to volunteer to take on the conscription.