Use the Analyze-Design-Develop-Implement-Evaluate cycle to understand and document processes such as business processes, usually with a goal of improving resource utilization or output. The whole cycle can be pursued entirely graphically until stakeholders agree the analysis has been fruitful.
Systems analysis as a business discipline had roots in the scientific management movement a century ago and today enjoys its highest achievements in the field of operations research.
Because the systems under analysis can be so varied, systems analysis has a variety of tools. Many of these tools are graphical, such as a process or information flow diagram, or dependency diagram.
The tools I use depend on the circumstances. Sometimes a cost/benefit analysis fills the bill. Sometimes it's worthwhile to build a computer model and run simulations. I usually start an analysis with graphical techniques, and I've usually found them sufficient for the analyses I've prepared.