- Sabotage one alarm bell
- Don't let Elise get detected
Go to le Champ de Mars
Follow Elise through the crowd until you trigger a cut scene. Your first objective is as simple as that.
Investigate Robespierre's Tent
Before you get started, look at your mini-map and take note of the shaded area. Those are the restricted ones, so as long as you're outside of them you can just walk around. The only exception to this is when you come across Extremists. Those fools will attack you at any time. Soldiers, however, will leave you alone unless you're somewhere you shouldn't be.
Head to the northeast until you're standing against a fence that is behind some tents and a tower. When the coast is clear, climb up into the tower, then move along the nearby ropes and trees until you're perched on a pole in the north corner of the restricted area. Ensure the coast is clear, then perform an Air Assassination on the guard below.
You should now see your objective marker in the nearby tent. Sneak through one of windows, then take note of the alarm bell outside. You need to sabotage it, and a Cherry Bomb might be just what you need to distract the two guards outside. Toss one, sabotage the bell, sneak back into the tent and grab the book off the table. This will complete your objective.
Speak to Elise
Leave the tent through the windows you entered, then hop the nearby fence to get outside of the restricted area. From here, make your way to Elise, using your Eagle Vision frequently to ensure no Extremists are around to mess with you.
Knock Out Approaching Guards
You have three waves of guards to be concerned with, and if they confront Elise you'll fail the objective, as well as the challenge.
As soon as you gain control, head to the hay cart to the northwest and along the fence. When a guard walks by, emerge and give him the Hidden Blade. The next guard comes from the northeast. Hide in the nearby crowd until he's close, then quickly approach and kill him. The last set comes from the southwest side, and there is another hay cart you can hide in. Once they pass, emerge and take them out with the Double Assassination skill.
Speak to Elise
Stay to the non-shaded area of the map and go speak to Elise.
Plant the Letters
After speaking with Elise, hop into the nearby hay cart. Just make sure the guard isn't looking, then perform a kill from your hiding spot when he returns. After this you can emerge, approaching your target with stealth and planting the letter on him.
Head to the objective to the northwest, walking along the safe zones and making sure to avoid any Extremists along the way. When you're close, hop the fence to your target's left, sneaking in the window and killing the guard at the main door with the Hidden Blade. When he's down, plant the letter on your target and then simply walk back to the safe zone.
Make your way to your third target, approaching him from the northeast. This one is complicated by the fact Extremists are in the area. We failed a couple of times, and even ended up killing half the guards in all of France in the process. What finally worked, however, was a Berserker Blade into an Extremist, which kicked off a battle between him and the guards. We then used the chaos to plant the letter and complete our objective.
If that doesn't work for you, consider tossing a Money Pouch at the feet of the Extremists. This should draw a crowd, and since you can walk openly in front of guards, it might be the window of opportunity you need.