Composition languages like BPEL and many enactment tools only support structured process models, while most composition approaches only consider unstructured models. In this paper, we outline a semi-automatic approach for composing a set of services with data flow dependencies into a structured process model. These data flow dependencies can be automatically derived from the input and output messages of each service, but some additional user input is needed to annotate dependencies with specific branching types. Heart of the approach is a fully automatic composition algorithm that given an annotated dependency graph constructs a structured composition. We illustrate the approach by applying it to an example case study from the CrossWork project, which studies the dynamic formation of cross-organisational workflows.