1.) Spend 2 whole days eating nothing but pure fiber and laxatives.
2.) Hold your shit in
3.) After the two days of making your shit into a pure liquid state, offer to have sex with your partner
4.) Then while having sex doggystyle tell your partner you are going to cum.
5.) Before she can turn around, you bend over and spread your cheeks.
6.) As soon as her head rears around you unload all of the shit you have been storing up for the past few days in one massive liquid blast all over her face.