Here is a link to a reddit post with all playable female rulers and their start dates and locations:
https://www.reddit.com/r/CrusaderKings/comments/2wmipk/an_ex... Pick one and marry a husband with as low an intrigue score as possible (matri-marrying doesn't matter since you'll be killing him off anyways). You can easily do this by clicking the button with a pair of wedding rings next to your character's portrait and then sorting by intrigue. There should be plenty of guys with zero intrigue. Once you're married, simply imprison and execute him. Do this two more times and the achievement is yours. This will piss of your vassals but they won't have time to do anything about it before you've got the achievement.