Once you have logged into Plesk you will be shown the client options section.
At the bottom of this section you will see your domain name(s), select the one you wish to add an alias to. This will then display the domain options, including email.
Choose email which will show you your current email addresses. To create an alias, you must already have an email account created.
Choose which email address you wish to create an alias for. Once you have clicked on the email address you will be shown the options for this email address, including Add New Mail Alias.
Select to create a new alias where it will ask you for the alias you wish to use, this must be unique. Once created all emails to the alias will be sent to the email you chosen at the start.