You start with a business model. The first step is to isolate the subjects. Basically every entity of your model is a candidate for a subject. However, a subject has typical properties that distinguish it from other entities:
If you have isolated the subjects, the next step is to associate activities with them. Begin to model the process for the main subject. Care about the other subjects later. The activities should be atomic units of work. Write down in textual form, what is happening within the activity. What are the preconditions, what side effects does execution imply and what postconditions are produced.
The modeller allows you to add a description to the process as well as to the activities. If you use this feature, the modeller will generate a html documentation from the process description.
The next step is to split up the textual preconditions and postconditions into separatly statable conditions and create condition nodes for them. Don't go into technical details yet. The modeller allows you to design your processes in a complete but non technical manner. The process description is the highlevel business view.
You may notice, that you need to store additional information with the subject (flags, dates, etc). You can either enhance the subject in your business model or put the process related information in a derived / related model class.