/u/gcronin is correct, the Popes absolutely did not ever lead troops in the Crusades, or even lay eyes on the Holy Land in most (all? Not 100% on this) cases. Their main role was to provide the theological impetus to crusade and the Church-based incentives to do so, typically by offering indulgences. Different popes took different approaches to their level of involvement. For example, in the First Crusade it's clear that Pope Urban II intended to play a much more hands-on role than he did. He had pre-selected an intended military commander, Raymond of St. Gilles, and a church leader to act as his legate, Adhemar of Le Puy. Unfortunately most of the other big names in the Crusade missed the memo on the whole unified front thing, and as a result you got first a totally unsanctioned force that was essentially a mob led by Peter the Hermit (slaughtered briefly after arrival in the Holy Land) and after that a more official representation but a very divided one, with Raymond of St. Gilles, Godfrey of Boullion and his relatives, both named Baldwin, Bohemond, and Hugh of Vermandois, brother of the king of France, plus big voices within those contingents like Tancred, Bohemond's nephew, Joscelyn of Courtenay, and on and on.

What you have there is a very diverse contingent that before it has even reached the Holy Land is well beyond the Pope's hope of controlling and is already off-message from the Crusade's original intent (Pope Urban gave a sermon in which he primarily focused on liberating Christians under Muslim rule, by the time the Crusader princes assembled in Byzantium the goal was the reconquest* of the Holy Land). He maintained a voice in the Crusade, but Urban's plans were disrupted essentially from the get-go. Godfrey of Boullion ends up as de facto leader of the contingent of princes, both military and otherwise, and while Raymond and Adhemar remained important players, the Crusade was no longer the Pope's operation.

That pretty much set the tone for Crusades, the Pope preaching it and giving religious incentives and the actual operations being carried out by secular powers, usually with secular motivations. One interesting possible demonstrator of the degree of power the Pope held might be the Baltic Crusades, conducted largely by the Teutonic Knights to Christianize and conquer Prussia. I have heard it argued that the Pope essentially changed the definition of what a Crusade was from being specific to the Holy Land to being Holy War waged anywhere for the protection of Christians, allowing the Teutonic Knights and others who participated in the Crusades to tap into church support and the sorts of theological rewards that were generally only available to Crusaders, as well as to be able to count the wars fought in Prussia as fulfilling the Crusading vow. I admit, however, that I don't have a lot of knowledge on that particular action or the lead-up to it.

The long and the short of it is: if the pope had to formally call it to be a Crusade. He was responsible for the theological impetus and ideology of crusading, but rarely had any influence over even general action and politics in the Holy Land, let alone planning specific operations. Most of the military action was carried out by nobility, the nobility governed the Latin States (the lands conquered by the crusaders in the First Crusade) and generally it fell to church officials acting on the Pope's behalf to represent the Pope's interest in the Holy Land.

Edit - Dur, sources, dur:

• The Age of the Crusades: The Near East from the Eleventh Century to 1517 - P.M. Holt
• The Crusades - Hans Eberhard Mayer
• The New Cambridge Medieval History - David Luscombe and Jonathan Riley-Smith, eds.

*To clarify on this, when I say "reconquest," that's how the Crusaders saw it, a reconquest for Christianity rather than something like, say, the Spanish Reconquista. The Franks, who comprised the vast majority of the Crusaders, had never held any territory in the Middle East to reconquer.