Every automation consists of a trigger, condition, and action. Action is actually the result of the fired trigger and completed conditions. Look at all actions:
Select the action after entering the name of your automation and selecting the project, trigger, and condition. There are some simple actions: Close issue, Close parent issue, Re-open issue. You just select them without any other details.
Other actions are more complicated: Assign Issue, Create issue, Create Subtask, Link Mentioned Issue, Log work, Notify Issue, Send Email, Transition issue status to, Update issue. You need to enter more details in opened forms.
You just need to select the assignee from the dropdown list and then select values in other dropdown lists.
Select and enter values into all mandatory fields.
Enter the title and description for your future subtask.
Link Mentioned Issue
You need to select the link type and then select the "Issue provided from the comment" or "Issue provided manually" value. For both of them, you'll see new fields. Enter or select values.
Here you may log work time to your ticket. Enter time, select the operations and then enter the description. You may select the work log visibility too.
Select users and enter the value for notifying users about any changes with this value.
Select the user for email and then enter the subject and content of your message.
Transition issue status to
Here you just need to select the "Issue transition to" value and you may select the "Chose To Set Field" value but it is not necessary.
Select the issue and validation option from dropdown lists. Enter the value and then select the way: add or replace. You may select the "Chose To Set Field" value but it is not necessary.
Click the "Continue" button after selecting the action and entering\selecting all values in fields. Then select trigger and condition. Enable the "Automation is ready - now turn it on" button. Automation is created and the trigger will fire after the conditions will be done on the board and then action will be done too.