Directly North of Megaton is the Meresti Train Yard. Slightly Northeast of the train yard is Agatha's house; look for a radio tower jutting out from some rocks. The house can only be reached via a rope bridge.
Agatha will often be asleep when you enter her home. Crouch down, and sneak up to her while she's sleeping. Make sure the "Hidden" message is displayed when you reach her. Click on Agatha, and transfer a grenade or frag mine from your inventory to hers. If done right, the explosive will show as "Live" before you transfer it.
To make it easier, use a Stealth Boy prior to sneaking.
Once the transfer is complete, the explosive will go off in a couple of seconds and the achievement will unlock.
Agatha is a quest-giver, so after you've blown her up be sure to reload your game after the achievement unlocks.