I've finished Leonardo's missions and he's given me a parachute but forgot to include the instructions.
Already read that you need to press X on an Xbox to deploy it but I'm playing on my PC and can't figure it out. I even died once, jumping from the highest tower, hoping it would deploy automagically :-)
There is only one consideration for the parachute to deploy. Ezio should be in freefall, not diving or in leap of faith. After you jump of a ledge, press and hold the attack button to deploy the parachute.
Simple: Open your game, then press the Esc button. Select the option "Database" and then select "User Manual". After that you'll see there are lots of options and click on your option - for instance "Parachute". It'll show how you can open the Parachute.