Developing Internet of Things (IoT) systems is not trivial and needs to be performed systematically to derive an IoT system that meets the required functional and non-functional concerns. Since IoT is applied to different heterogeneous domains usually a one-size-fits-all method is less feasible. For some cases a lightweight method with a few method artefacts are sufficient while in other cases a detailed set of method artefacts over the whole lifecycle might be required. So far, a few IoT system development methods (SDM) have been provided that include the steps necessary for guiding the development of IoT systems but these do not explicitly consider the situational needs for the required IoT method. In this paper we propose a situational method engineering (SME) approach for developing a method base that includes a broad set of method fragments which can be reused to develop customized methods. We illustrate the development of the method base using the existing IoT methods that have been proposed in the literature so far. Further we show how the method base can be used to develop methods for two different cases.