...Despite the availability of general design guidelines in the literature, clear and well-established procedures for the optimal design of mid-air gesture-based interfaces are, to date, not available and still remain an open issue. The main contribution of this work is a user-centered and extensible framework, which integrates existing and novel methods. It supports the mid-air interface designer considering multiple aspects including ergonomics, memorability, and specific user requirements tailored to the application scenario. The framework involves three design steps and a final validation step, also supported by a dedicated software. ...