Login to your reseller client account with your username and password.

In the Domains section click on "Create" to create domain.

Give the domain name,select Hosting Type and give the Hosting Setup Settings i.e. FTP username and password

and then click 'Next". 

Here we provide the hard disk quota for the website, select the application support we want to enable on the site

and click on "Next".

Now we set the total resource usage limits for the domain and click on "Finish"  to create the domain.

After creating the domain  you require to change the nameserver settings pointing it to the server  through which your domain is  currently running, for that you need to change the  nameserver records from DNS settings in your domain.

You can refer to the below tutorial for changing DNS records.

http://kb.diadem.co.in/idx.php/19/068/Plesk-Tutorials/article/Creating-and-Deleting-DNS-Records.html

Logout after you have finished.