Actual advice: Strangulation is easier and quieter. If you’re looking to chop her up, you’ll also have her in better condition if you strangle her, as a stabbing would require a very precise hit to the heart or lungs to kill quickly. A stab in the back might not even be fatal, or might lead to a very slow death, also depends on the length of the knife you are using. In contrast, strangulation can be done with just your bare hands, or a rope/cloth if you’re not as physically strong. Also, unless you are very dedicated to your craft, I would not recommend buying a coffin. This girl’s disappearance could possibly point to you as a suspect, and having bought a coffin will look very sus. If you’re going to chop her up, you could feed her to animals (pigs work best if you know of a farm), burn the pieces and scatter the ashes, or dissolve them in acid. Note that if you choose to take the acid route, don’t dispose of the acid down your sink/drain. It will corrode your pipes and may result in plumbing issues later that are expensive to fix.