The top-down formation of a fullerene from a graphene flake is investigated via extensive ab initio molecular dynamics simulations in the range 300-3000 K, accelerated by metadynamics. Topological (SPRINT) coordinates are used to ensure a prejudice-free exploration of the free-energy surface and path collective variables to provide reliable free-energy barriers. The low-barrier zipping of the 2D nanoflake into a 3D nanocone is revealed as the early key transformation, mediated by a four-membered ring. Multiple-step pathways lead it toward different but always fully tricoordinated OD closed cages. This scenario comprises several key chemical reactions characteristic of carbon at the nanoscale, as known from diverse experiments.