First you need to access your email control panel. You can read how to do this in the How to access your email control panel KB.

From the main email services menu for your desired domain. You select the webmail option, as shown in the screenshot below.

Once you have selected the webmail option. You will be taken to a screen where all the email addresses you have created for that domain are shown. Next to each one, is the - sign in to webmail - option.

Select the one you want to access webmail for and you will be automatically signed in to that email accounts webmail. From here you can send, receive and administrate all your email as normal.

Remember there is also the option to forward your email to another email address using the email forwarders function as well.

Please note. It can take upto 12 hours after the initial creation of an email account, before you can recieve email. As explained in this KB article.