You will need the Full Write Workflows permission to perform this action.
Click on "+ New" in the top header area and then select "Workflow." You can also click on Automation > Workflows from the left-hand navigation menu and then click on "Add New" at the top.
A pop up will appear with the following fields:
Name - the name of the workflow.
Description - a brief description of what this workflow does.
Note: the workflow will be disabled initially. This is to prevent any automation from occurring while you are designing it. Once you are done and are ready for the system to start doing the automation, edit the workflow and enable it.
Once you are done, click on "Create" to add the workflow. You will be redirected to the edit workflow screen.
Security
See this article for information on controlling access to the workflow. The user who creates the workflow will also be added as an admin on the workflow.