Use the Lasso tool ( L ) or Polygonal Lasso tool to rough-in a shape. After making your selection, open the Paths panel (Window>Paths), press-and-hold the Option (PC: Alt) key, and click the Make Work Path From Selection icon at the bottom of the panel. Choose a suitable Tolerance from the ensuing dialog and click OK to create the path. Grab the Direct Selection tool (nested under the Path Selection [ A ] tool in the Toolbox) and edit the path as necessary to achieve the proper geometry and smoothness. Control-click (PC: Right-click) the path and select Define Custom Shape, name it, and click OK.