I will explain how to do concrete computations in bosonic closed string field theory. For this, I will show how to numerically describe the geometry of the four-point contact interaction, by computing the boundary of the relevant region of the moduli space of the four-punctured spheres, and by computing everywhere in this region the local coordinates around each punctures, in terms of a Strebel quadratic differential and mapping radii. I will then explain how these results are used and checked in the recent paper of Yang and Zwiebach by considering marginal fields in closed bosonic string field theory.