Customising offerings effectively for customers’ needs and wants, yet maintaining the provider’s efficiency, is a challenging task. Modularisation is a key to achieve this, but insight on how to create modules for customising product/service systems (PSSs) is limited. This article proposes a new practical method that supports designers to create service modules by extending the DSM (Design Structure Matrix). The method has been applied to existing PSSs by an elevator manufacturer. It was found effective because it reduces complexity in customisation, and thus response time in service provision, as well as increases the chance of reusing previous designs.