Here are the key steps to cost a WBS:
1. Estimate the cost of each work package or activity at the lowest level of the WBS using estimating techniques like analogous, parametric, bottom-up etc.
2. Assign resources like labor, materials, equipment to each activity and apply the appropriate cost rates.
3. Use project management software to automatically sum or "roll up" the costs of individual activities to higher levels and the total project cost.
4. The software allows viewing and reporting the costs against the WBS structure to identify areas that may need attention or re-estimating.
5. The costed WBS provides the basis for developing the project budget and cost baseline for monitoring