It all starts at the shoulders, as this is the widest aspect of the body and the beginning of the V-taper. More specifically, we're talking about the middle deltoids, which are the lateral "caps" of the shoulders; the front (anterior) and rear (posterior) delts provide thickness to the shoulders, but not width. Your go-to exercises here are mass-building overhead shoulder presses, lateral raises, and upright rows, all of which hit the middle delts.
Next, there's the chest. When it comes to creating a V-taper, not to mention an X-frame, the chest doesn't quite deserve the priority of the shoulders, but well-developed upper and outer pecs certainly add to the aesthetic by adding some width and creating the appearance of "broadness" (for lack of a better word). The best exercises for this are incline presses and flye movements.
The latissimus dorsi (lats) are as important as any muscle when it comes to creating maximum width in the upper body. When someone is said to have a great V-taper, wide lats are always present. And what exercises develop wider lats? Pretty much anything with a wide grip – namely, wide-grip pull-ups and wide-grip lat pulldowns, though rows performed with a wide grip are useful as well.
Finally, the bottom half of the X. Overall quadriceps development is important, but there's one specific quad muscle you want to focus on for wider thighs: the vastus lateralis. This is the muscle responsible for the "outer quad sweep," where the legs bow out from the waist to create lower body width. Targeting this area involves doing squatting movements with a narrow foot stance (inside shoulder width). It's impossible to fully isolate any one quadriceps muscle from the other three (vastus medialis, vastus intermedius, rectus femoris), but narrow-stance squats will nonetheless emphasize the lateralis.
Now that you know the V-taper and X-frame muscles, it's time to train them with the below exercises...