1. Add a mashable or other mashup as the source of external information for this mashup. Or use DirectInvoke, SQL or a custom block to supply this information.
2. If needed, use other actions to filter or transform these results. This may be needed to meet your requirements for the data you want in SharePoint or to meet Data Requirements for the Results of ECT-Enabled Mashups such as limiting the number of items for the list based on this mashup.
3. Add an Input block and change the name for this input to match the name of the field in the information source that is the ID for each row.