Here are the key steps to create a mapping in Informatica PowerCenter:
1. Open the Mapping Designer and create a new mapping
2. Drag and drop the source and target tables from the Repository Navigator into the mapping area
3. Create an Expression Transformation and name it appropriately
4. Connect the source table ports to the Expression Transformation ports
5. Right click the Expression Transformation and select 'Edit' to open the Expression Editor
6. In the Expression Editor, add a dummy output port for the field to be calculated/transformed
7. Write the expression in the Expression Editor to calculate/transform the field value based on the business logic
8. Connect the Expression Transformation output port