Sign up for an Office 365 Developer Site

Be sure you have access to an Office 365 Developer Site. Here are three ways to get one:
Please refer to Sign up for an Office 365 Developer Subscription and set up your tools and environment for more details.
Install SharePoint Designer 2013
SharePoint Designer 2013 is a free download. To download and install SharePoint Designer 2013 use the following procedure.
To install SharePoint Designer 2013
  1. Open your web browser and navigate to the Microsoft Download Center.
  2. Click on the Download button.
    Download
  3. Select your platform type: 64-bit (x64) or 32-bit (x86). Click on the Next button.
    Select your platform
  4. Follow the instructions to install SharePoint Designer 2013.
    SharePoint Designer
The following is the procedure to connect to a SharePoint Online site in SharePoint Designer 2013.
  1. Open SharePoint Designer 2013.
  2. Click on Open Site.
    Open Site
  3. Enter the site URL and then click on the Open button.
    Enter the site URL
  4. Enter your credentials, if prompted. Be sure to use credentials that have access to the SharePoint 2013 site.
  5. The site is opened successfully in the SharePoint Designer 2013.
The following is the procedure to create a list workflow using SharePoint Designer 2013.
  1. Click on the Workflows link in the Navigation Pane.
    Click on Workflows
  2. Click on the List Workflow button in the interface and then click on the list to which the workflow should be added.
    Click on List Workflow
  3. Enter the Name and Description of the workflow.
    Description
  4. Select SharePoint 2013 as a platform type.
  5. Click on the Ok button.
  6. Click on Transition to stage and then click on the Action button in the ribbon interface.
  7. Click on Go to a stage.
    Transition to stage
  8. Select End of Workflow.
    End of Workflow
  9. Click on the Publish button in the interface.
    Publish button
  10. The workflow will be saved and published to the SharePoint 2013 list.
    published
The following is the procedure to test the workflow.
  1. Navigate to the Office 365 developer site.
  2. Navigate to the SharePoint list to which the workflow is added.
  3. Click on the List tab and then click on the Workflow Settings.
    Workflow Settings
  4. You will see that the workflow was added to the list successfully.
  5. Navigate to the list and select an item.
  6. Click on Advanced and then click on Workflows in the ECB menu.
    Workflows
  7. Click on List Workflow.
    List Workflow
  8. After a few minutes, the workflow is completed successfully as shown below. (Note: I have not added any actions to this workflow, it was created to simply show the procedure for creating a list workflow.)
    creating a list workflow

Summary

Thus in this article, you saw how to create a list workflow in SharePoint 2013 Online.