One side or the centre
A single design on one side of the chest, above the pec, is the classic placement and leaves room to add a matching piece on the other side later. The sternum and the line under the collarbones suit long or symmetrical designs.
Chest skin stretches and changes shape with the muscle and with weight, and the area over the sternum is thin, which many people find among the more painful spots. Keep the most important detail on the flatter part of the chest.
Fitting it in the editor
Stand straight and face the camera, arms relaxed, in even light. Flat works for most of the chest; for the curve of the pec, bend the edges slightly with Free warp. Try the design at several sizes: the chest can take more than you expect.