The steelpan is a tuned percussion instrument from Trinidad and Tobago. It is made of oil barrels that are subjected to several stages of metal forming that stretch and bend the structure. The top of the barrel is pressed, hammered, punched and burnt in order to obtain a sort of main bowl within which convex substructures are formed. Due to the thickness of the metallic structure, geometric nonlinear vibrations are a key feature of the peculiar tone of the instrument. We are interested in developing a numerical model accounting for all the transformations encountered by the structure during the steelpan making. Simultaneously, a real pan is built by a panmaker and numerous measurements (geometry, modal analysis, stress measurements) are realized after each step. The first step (the sinking) has thus been studied. A special attention is paid on the effects of the prestresses on dynamical properties of the thin curved structure. Analytical-numerical models with simplified geometry (beams and plates) are used to quantify the effects of the prestresses on the eigenfrequencies. Then, a finite element model is derived for a quantitative comparison with measurements at several surface points. All these data allows fine modeling of the know-how of the pan maker for the sinking.