A method of making a modular wind turbine blade is described. The modular blade comprises first and second blade modules connected together by a scarf joint between tapered spar caps of the respective blade modules. According to the method, first and second blade modules are laid up in the same mould assembly. A separating layer is arranged between the layups of the first and second module in a joint region of the mould. The separating layer has a thickness corresponding to a required bond thickness in the scarf joint when the modules are bonded together.