First thing you want to do is go to the closest ammo dump vending machine and purchase a grande mod that specifically has Sticky listed on the traits list and equip the mod from your inventory. You should then go out into the wilderness to find a group of weak enemy's (for example I found a group of skags). throw your grande into the general area of the enemy's (this does not not need to stick onto one of the enemy's) wait for the grenade to go off and your achievement should pop. If it doesn't then try again until it does.
You can damage enemy's and still get the achievement as long as the grenade gets the kill.
As noted in the description you can get more than two kills from the grenade you just need to make sure it kills at least two.
If you would like to see a video guide for this please see the one I created below.