Assassin's Creed II/Sequence 4: The Pazzi Conspiracy

Practice what you preach
Leonardo will translate the Codex page you got from Vieri, and he'll be able to upgrade your hidden blade. In the meantime, you'll need to practice the new assassination techniques on the dummies set up outside the workshop. Head into the coutyard adjacent to the workshop to get started (you'll see three red target indicators on your map. You can perform the assassinations in any order, and as many times as you like: You can practice on the dummies as long as you want, and when you're done just head back into the workshop. Leonardo will give you back your hidden blade with the improvements he's made and he'll tell you to talk with La Volpe to find a way to your next target.
 * Memory start: Knock on the door to Leonardo's workshop.
 * Reward: 1,000ƒ, Air assassination skill, Assassinate from hiding skill, Second Hidden Blade, Races unlocked, Assassinate from ledge skill, Hidden Blade armor plate
 * 1) Hide in the hay cart and press the armed hand button to learn "Assassinate from a hiding spot".
 * 2) Climb up to the railing in front of the dummy high above and press the armed hand button when you're in front of him to learn "Assassinate from ledge".
 * 3) Climb up to the ledge above the target dummy in the back of the courtyard and press the armed hand button to learn "Air assassinate".

Fox hunt
When the mission starts you'll see a circular area marked in green on the map, which means your target is located somewhere within. Usually you'll need to use eagle vision to locate your target in gold among the people present, but for this memory, you just need to get close to him. Head around to the opposite side of the market and you'll find man standing alone by one of the pillars on the end. When you get close enough to him, a cut scene will trigger with a thief picking your pocket and running way. Run after him and tackle him by pressing the unarmed hand button when you get close enough. He runs pretty slowly compared to you, so this shouldn't be a problem. When you've caught him, you'll see a cut scene where you meet La Volpe and ask him to help you find Francesco de' Pazzi.
 * Memory start: Enter the white marker outside the Mercato Vecchio.
 * Reward: 1,500ƒ

See you there
Follow La Volpe as he runs through the city to Santa Maria Novella. Start out by climbing up the same way he went to the first white marker. After that, you'll have one minute to get across the district to your destination. It's a fairly direct shot across the rooftops; just remember to climb up to a higher roof when it looks like you'll have to jump down or go a different way. Eventually you'll end up on a rooftop next to Santa Maria Novella, where La Volpe will teach you how to get into secret catacombs. This skill is used to enter assassin tombs and collect seals to unlock Altaïr's armor back in the Sanctuary at your Villa, but the first one is part of the next memory.
 * Memory start: Talk to La Volpe at the top of the steps to the dock, just below where the last memory ended.
 * Reward: 1,200ƒ

Novella's secret
Don't bother distracting the guards to get into the area with the secret entrance – just climb the wall right beside them to get in. You'll find yourself in the catacombs beneath the Santa Maria Novella. This is a restricted area, as are all assassin tombs, which means you're automatically notorious and guards will attack on sight. Head down the steps in front of you and pull the lever on the right to lower the block at the far side of the room. Climb up and out the way you just opened, and go through the next room out to a staircase and a cut scene showing you how to proceed.
 * Memory start: At the secret entrance in the guarded corridor on the south side of Santa Maria Novella.
 * Reward:

Head directly ahead down the broken staircase and free run to swing across to a beam and over to another ledge. Keep free running in that direction over some ascending beams that end with a swing to a final wooden beam over a wooden platform. From there you can jump to the ledge up above and pull the lever up there. This will raise the gate in the hallway inside the structure in the middle of the area, so jump over to it and run through to the ledge on the other side. Over there you'll be above the parts you were free running earlier, and you can head across some more beams and swing bars in the same counterclockwise direction to a final ledge with another lever.