The first time you pick up stickies is in Chapter 6, and you can get the achievement as soon as you pick them up if you're careful. You simply have to aim and toss it at an enemy and hit him. You can hold to view where the grenade will be tossed. Once you hit him, he will kind of do a little panic dance and then blow up.
During Chapter 7, there is a guaranteed spot to get this one. You will eventually come to part where you have to shoot out a big glass in order to continue. After that you rappel down into a desert area, and you will come across several guys standing around, not noticing you. Before firing at them, look for a sticky grenade crate right as you enter the area. Crouch down and aim the grenade at one of the guys, making sure to stick him.