To unlock the Spit Roast trophy in Assassin's Creed III Remastered, you will need to perform a double assassination using a musket. You can do this at the start of Sequence when you leave the ship.
First, you will need to obtain a musket by either killing a guard or finding one. Make sure the musket is loaded, and then find a couple of guards that aren't aware of your presence and position yourself so that they are both stood in a line in front of you.
Hold
so that the closest enemy is outlined in white and the one furthest away has a white shimmer, and then press
to perform the double assassination.