Nanoscopic vibrating beams and membranes constitute some of our most precise tools for measuring minute physical forces, a precision enabled by a combination of low inertia (i.e. low mass) and long coherence times (i.e. high mechanical quality factors). Nanomechanical resonators made from two-dimensional materials, like graphene, represent the ultimate limit for low mass. However, their quality factors have remained orders of magnitude lower than in similar bulk systems, especially at room temperature. Furthermore, no unified theory has been rigorously tested to explain this discrepancy, hindering efforts to engineer higher quality factors. In this talk, I will describe our efforts to understand the quality factor in graphene nanomechanical resonators using the elastic theory of dissipation dilution. Applying this theory, we can tailor the shape and strain of the graphene mechanical resonators using focused ion beam irradiation and improve their quality factors to values exceeding 15,000, the highest measured to date.