Select "Update Value" action type (3), click "Add" button (4).

'Update Value' actions perform updates of following entities:

• Form - representing list of form elements that can be updated
• Variables - form variables
• Context - context object of the form (e.g. "Case")

Source for the update can be of one of the following types:

• Parameters - parameters passed to the action
• Form - form elements values
• Variables - form variables
• Context - context object of the form
• Static - static value provided by user

Select target context and field/variable to be updated (5).
Select source context and field/value to update target with (6).

"Global Action"/"Global Action Group" actions trigger global action/group execution and allow you to use the results of execution in form config.

Select proper type in "Type" picklist and populate other fields:

Name (7) - click Settings icon (8) and select an action/group. Click Arrow icon (9) to open selected action in new tab (if no action/group is selected yet, an icon will be "+" and will open new action/group creation form).

Source (10, optional) is id of a record that is used as a data source for global action. You can set it directly of configure with merge field constructor (click icon 11).

Set variables (12) if you need to pass parameters to global action. Follow the sequence of step to add and set variables in the popup appeared:

Result Handlers are form actions that are executed once global action execution is done. Click icon (13) to select and configure them (they should be configured at the form first).

You can pass parameters to Result Handler actions if necessary - click Settings icon (15) for setup. Follow the sequence of steps to add set parameters in the popup appeared.

"Global Action Group" form action setup is same as "Global Action".
Turn Checkbox on to disable spinner that is shown by default while action is executing.

For more information about usage of Global Actions and Global Action Groups please refer to the following documentation: Actions/Action Groups

'Refresh' actions perform refresh of one of the following form elements:

• Form itself
• Lists
• Related Lists
• Lookups
• Nested Forms

Select "Refresh" action type (16) and select target element of form (17) to be refreshed:

Navigate action type allows user to navigate to one of the following resources:

• URL
• Visualforce Page
• Record
• Form
• Lightning Component
• Return Back

Navigate to URL:

Configure URL to navigate to (18).
Choose if open in new window (19).
Pass parameters if necessary (20).

Navigate to Visualforce Page:

Select page to navigate to (21).
Choose if open in new window (22).
Pass parameters if necessary (23).

Navigate to Record:

Type record ID to navigate to (24) or configure it with merge field picker (25)

Navigate to Form:

Pick the form to navigate to (26).
Choose form mode (27).
Pass parameters if necessary (28).

Navigate to Lightning Component:

Pick the component to navigate to (29).
Choose if it should be opened in popup (30).
Pass parameters if necessary (31).

Return back:

Pick "Return Back" (32).
Specify number of history steps to go back (33).

"Form API" actions execute one of the following methods:

• Save - saves context object
• Reset - resets the original values of all fields in a form
• Clear - removes values from all form inputs
• Show Notification - displays notification to user
• Validate - validates form inputs and shows error if some of them are blank
• Print Form - allows to create a PDF version of the Form, which can be printed
• Fire Event - fires lightning event. It can fire aura application event or force event
• Execute Form Actions - runs action (or group of actions) on the current form/nested form/parent form
• Close - closes current form
• Calculate - performs calculations/executes formula on the form (available from version 5.0)
• Update Process Instance Variables - allows to update variables in specified Process Instance (available from version 6.0)

Select necessary method in the "Form Action Type" picklist (34).

Save:

Check "Disable Validation" checkbox (35) if you need save data that does not match form validation rules.
Select type of Success Message (36). If you want to get rid of it, select "None" (available from version 5.0). To override default message select "Custom Message" and configure it (37).
Configure Extended Fields (38) if needed (available from version 5.0).

Show Notification:

Select Notification Type (39).
Populate or configure "Title" (40) and "Text" (41).

Validate:

Select where validation should be performed (42).
Configure "Message" (43) if you need to inform users.

Fire Event:

Populate Event name (in namespace:name format) (44). Fire Event action example (force event).
Specify event parameters (45) if needed.  

Execute Form Actions:

Specify Timeout (46) - it is optional setting to delay form action execution.
Select "Form" (47) and click "Add" button (48).
Based on what form is selected (current form or nested form), "Action" will display list of current form actions or nested form actions (49).
Configure parameters if needed (50).

If you check "All Forms" (51) - you can execute action from any form in your organization.

Calculate Form Actions:

Configure calculation expressions. Specify output variable name (52) and expression which evaluation result will be a value of output variable (53). You can configure several output variables.

Results of calculations should be passed to some other action (result handler) (54).

Configure parameters of result handlers (55) and select action (action should be configured at the form first).

 Handler action updates form field with the calculation result:

Update Process Instance Variables (available from version 6.0):

• Select Process Instance from the list (58) by clicking button (56) or use editor (55) to type Process Instance ID or select Merge Field
• Specify Variables defined on that Process Instance, and their values (57)