Start by taking a systems view of your task.

Approach with a simple overview.

Think of an activity diagram or UML activity diagram as a way to show the flow or sequence of actions that are carried out in a system - modeling how a system's actions are performed to complete a process.

Activities can be laid out sequentially or concurrently depending on the flow or sequence of actions.

When getting started, first focus on: 

• Initial state and final state – All activity diagrams have an initial state and final state that mark the start and end of the process.
• Activity or action state – Represents a single activity that sets a series of actions into motion (i.e. issue license).
• Actions – An action or step in the activity in a task is performed (i.e. driver's license test).

Know the scope of your activity.

Have a beginning and ending in mind.

The first step is to identify a process or activity that you want to visualize and outline the steps or actions that are required for its completion. 

Create the initial node as the start of the process. Next, use activity or action nodes  in sequence - creating a flow.

Add objects, decisions, signals, and concurrent activities.

Connect all the steps together using action flows and  mark the end of the process with an end node.

Additionally, you may need:

• Objects – Materials or data that are created or used within a process.
• Decisions – Questions that need to be answered before proceeding to another action or activity.
• Synchronization – Use a fork and join node to create concurrent flows and then merge back the flows  into a single flow, respectively.
• Signals – Indicates actions that are outside the system.
• Swimlanes – Group related activities that are carried out by different actors.