Start by clicking the Gear Icon in the upper left and Selecting "Setup"
In the search box under "Setup", type Object Manager and then click the "Object Manager" link
Navigate to the "Account" object and click on the label "Account"
On the Account Object Manager page select the "Page Layouts" link on the left hand menu. Select the Layout where you would like to add the Action. Please note you will have to update all of the layouts that you would like the the Quick Action to be available.
Once you select a layout, navigate to "Mobile & Lightning Actions" in the canvas items and drag the "Add Credit Note" Quick Action to the "Salesforce Mobile and Lightning Experience Actions" section. Order the Quick Action as you would like for your button list. Click the "Save" button to finish your changes. NOTE: The "Salesforce Mobile and Lightning Experience Actions" may not be enabled on your selected layout. In order to enable that section click the wrench Icon on the far right of the panel bellow that section title.
Navigate back to any account detail page to verify the Action is added to the page. If you do not see the button, click the down arrow at the end of the button row to make sure it is not in the overflow buttons. If you still do not see the button make sure the pay layout you are viewing is the correct one and has the Action saved to the layout.
From any Account Detail Page, click the gear icon on the upper right and then select "Edit Page"
From the Standard Components list on the left, select "Flow" and drag it onto the canvas where you would like it. In this example I chose the right panel of the page. When placed on the page, click the component and set the Flow name on the right to "Add Credit Note" and make sure the "Pass record ID as variable" checkbox is clicked at the bottom. Click "Save" at the top right to save this lightning page.
After saving your changes, navigate back to an Account Detail screen in order to validate the component is there and active.
This managed package includes a permission set that should be aded to any users for which the Credit Note Widget was not originally installed for. Navigate to the User that needs access to the Widget and click "Edit Assignments" in the "Permission Set Assignments" section. Move the "Credit Note Widget" permission set to the "Enabled Permission Sets" side and click "Save".
The Beta Release for this Widget.