There really isn't a way that we can give you explicit instructions on how to do this. We can give you vague ones, because we cannot see your system.
Go to: Admin panel -> Agents
Click Add New Agent
Populate at least the First Name, Last Name, Email address fields.
Click on the Access tab
Set the Primary Department, and the Role you want them to have.
Under Extended Access add every department you have configured.
Ensure that you are happy with the settings under the Permissions tab and Teams tab.
Click Create