The Statement of Work (SOW) should capture the tasks that the contractor has to perform under the contract. For service contracts, the Performance Work Statement (PWS) serves as the SOW and the scope of work should be performance-based to the maximum extent possible where the tasks are written by describing the scope in terms of the required results instead of telling the contractor how to perform. There is an optional handbook MIL-HDBK-245 that has served DoD well over the years with writing SOWs that may help you in this endeavor. The below website has the link for this optional handbook. 

https://www.dau.edu/cop/dm/dau%20sponsored%20documents/forms/reference.aspx