Grenades are just like rockets, except you have to wait for them to explode. =P The trickiest thing about them is 1)timing with the explosion and 2)your position when it explodes.

Timing. The easiest way I learned with these is to throw a grenade straight down and just listen to the bounces. 2 bounces, and then an explosion. However, when you shoot one straight up, you get a long pause, a bounce, then an explosion. Listen to the bounces! They really do help. Online grenades seem, for me, to explode faster by a split second. Having said that, offline grenades seem to take longer to explode. For all other grenade timings other than straight down and straight up, just toss a grenade and listen for the timing when it explodes. It's easiest to just toss empty grenades at the same spot until you know you've gotten down the timing for the explosion.

Grenade Positioning. When you point down and shoot a grenade, it doesn't exactly fall straight down with the floor. It's slightly tilted upward. So when you shoot it, it bounces twice, but is now positioned a little further away from you. Take this into account when trying to position grenades near edges. If you find your grenade falling off ledges, walk to the edge, stand sideways and shoot it.