The seventh crusade, led by King Louis IX, was three years in preparation, due largely to the fact that Louis wanted to be more thorough than the many failed crusades that had come before him. This seemed to do him little good, as he merely went down as the only European monarch to be captured during the course of the crusades. Since Louis was a famous king, and this crusade took place in the 13th century (when recorded histories were more commonplace), there is a good amount of information regarding the preparations that Louis made.
Through all his maneuvering raising money, raising an army, and finding transportation, Louis angered only one faction; the Venetians were displeased that he neglected them, instead choosing to get his transport ships from Genoa and Marseilles. Their displeasure was rather futile, however, as Louis was far to powerful a monarch for them to have any hope of challenging.
After spending three years settling foreign affairs and amassing his army (primarily French, though not without a handful soldiers from the other European nations), King Louis was ready to slaughter some heathens. He set sail for Cyprus, where he was greeted by a number of Palestinian barons, as well as the leaders of the Hospitalers and Knights Templar. Following the logic of the fifth crusade, Egypt was set as the target for the initial attack.
Once the unpredictable storms of winter had drawn to a close, Louis set sail for Egypt, though he had boats enough only for one fourth of his army. The Egyptians were aware of Louis' intentions, and so had stationed a force to oppose the crusaders, but the king refused to be deterred. A short skirmish between the Christian forces and the Egyptian defenses ensued, resulting in an Egyptian retreat at nightfall.
Damietta was the next major point of defense on the path to Cairo, and so it was to this city that the Egyptian troops retreated. After reaching Damietta, the Egyptians decided to retreat further, and though the reasons for their actions cannot now be known, the most likely explanation is that the troops were demoralized, and so their commander saw that they would only be routed if they made a stand right then. And so, without opposition, the crusaders took Damietta, the stronghold that temporarily gave the Europeans the advantage in the fifth crusade.
Stationed in Damietta, the crusaders were faced with a delay; the Nile River would soon be flooding, and they did not want to be caught on a flooded field of battle. After a few months, when the water levels had sufficiently receded, the crusaders set their sights on a camp just outside the city of Mansourah, the next Egyptian line of defense. The only difficulty was that, in order to attack, the Christian army had to cross the Bahr as-Saghir River. They unsuccessfully tried to build a bridge, and were unable to cross until a Egyptian citizen offered to show them a ford across the river.
King Louis' brother led the first group of French troops onto the other side of the river. The king had given explicit orders not to attack until the full force was together, but his brother feared being discovered by the Egyptians and so attacked immediately. The initial attack was successful; unaware that the Europeans had crossed the river, the Egyptians scattered, mostly retreating into Mansourah where they could mount a decent defense.
Seeing how great his initial success was, the king's brother immediately led his troops into Mansourah, confident that the city would fall easily. The Egyptians who had retreated allowed the crusaders to charge into the city, making it appear undefended. Then, when the Europeans could not escape, they launched a devastating counterattack. In the ensuing slaughter, the king's brother was slain, as was the Earl of Salisbury, Lord of Councy, and Count of Brienne. Grand Master William of the Knights Templar escaped, but lost an eye in the battle.
When Louis learned what was going on, he braced for an Egyptian attack, but his archers had yet to cross to river and, without them, the Egyptians were a formidable force. Most sources give Louis credit for keeping his soldiers together until the archers arrived, but I find it much more likely that it was the collective actions of the Europeans, who likely knew that they were fighting for their lives. Whatever the reasons, the crusaders were able to hold out until the archers arrived, at which point the Egyptians retreated to regroup.
Louis had won that battle, but was still aware that he had the weaker defensive position. When he tried to retreat back across the river, he was waylaid by Egyptians and his entire army was taken hostage. After Queen Margaret paid his ransom, Louis traveled to Acre (in Outremer), where he spent several years handling affairs as something of a monarch of Outremer pro tem. Eventually, following the death of his mother, he returned to France and resumed his rule, disgraced by his failed crusade.