Short answer: letters, written by the pope (or a king, or another secular or church leader), along with preachers being sent out by the church or being instructed (also by letter) to preach to everyone from the highest kings to the smallest parish churches.

In 1095, Pope Urban II started the preaching himself at the Council of Clermont. This is sort of presented in the sources from the time as an almost spontaneous event that no one knew about beforehand, but it must have all been planned in advance - local French nobles and bishops were all present, and Adhemar, the Bishop of Le Puy, was the first to volunteer to go on crusade. He then returned to his diocese and preached the crusade there, as did other bishops in their home dioceses. The council was probably also coordinated beforehand with the local secular rulers - Raymond IV of Toulouse, the most powerful ruler in southern France, was also among the first to sign up.

Urban stayed in France after the council and travelled around preaching the crusade himself. He also sent a letter “to all the faithful” in Flanders, as well as other letters to the Italian city-states like Pisa and Genoa. Sometimes the news preceded Urban and his letters - the idea turned out to be so exciting that rumours spread faster than the preachers.

Later crusades were also spread by preachers and letters. News of the Second Crusade was spread in a papal letter, “Quantum praedecessores” and the Third Crusade in “Audita tremendi” (after the first two words of the letters in Latin). The most famous preacher for the Second Crusade was the Cistercian monk Bernard of Clairvaux, who personally recruited the kings of France and Germany.

There weren’t really many English people on the First Crusade, no one significant anyway. Crusade preaching was relatively rare in England at first. England participated in the later crusades though, especially the Third, which was led by Richard the Lionheart, and the crusade of Prince Edward (later king Edward I) in 1270. Just before the Third Crusade, the Latin Catholic Patriarch of Jerusalem arrived in France and England, begging for support for the crusader states from western leaders. For the Third Crusade, the Archbishop of Canterbury preached the crusade in Wales, as recorded by Gerard of Wales. In the 13th century, the Dominican Order was founded. They were known as the “Order of Preachers”, and often what they were preaching were calls for crusades, in England and in the rest of Europe.

We don’t typically know what “common” people thought about anything, even crusading - although following the preaching for the First Crusade, there was a “people’s crusade” that made it all the way to Anatolia before being slaughtered by the Turks. For later crusades the church tried to prevent people from going if they weren’t knights or had no experience with warfare but there were probably some commoners following along as well. People who couldn’t fight, but who could afford to support the crusade effort, often pledged money instead of participating in person. In return, they would receive the same spiritual rewards as those who did go in person. While the Archbishop of Canterbury was preaching in Wales in 1188, one Welsh man pledged 20% of his belongings to the crusade. Dominican preachers in the 13th century were money collectors as much as anything else - they collected funds for the crusade if people couldn’t be convinced to travel to the east.

I don’t know if there are any surviving accounts of a parish priest specifically preaching to his local parish, but there are lots of preaching templates, things that a priest or bishop could say in their own territories so that the message would be consistent across Europe. The church instructed preachers what to do and say to convince people to join (or to give money instead).

So, people learned that a crusade had been called when the Pope sent letters to local bishops, or showed up preaching the crusade himself; the bishops and priests received these letters and then went out and preached in their home territories. The church also sent out its own preachers who were specifically trained for the job. If we’re talking about England in particular, it often wasn’t a target for crusade preaching, especially not in the early years, but for later crusades the same letters and preachers arrived in England just as they did in France or anywhere else in Europe.

Sources:

Jonathan Riley-Smith, The First Crusade and the Idea of Crusading (University of Pennsylvania Press, 1986)

Simon D. Lloyd, English Society and the Crusade, 1216-1307 (Oxford University Press, 1988)

Penny J. Cole, The Preaching of the Crusades to the Holy Land, 1095-1270 (Medieval Academy of America, 1991)

Christopher Tyerman, England and the Crusades, 1095-1588 (University of Chicago Press, 1988)