Click the Setting cog, top right » settings » Accounts and Import
"Check mail from other accounts", click "Add a mail account"
In the new window, type in your full email@example.com email, click "Next"
Use the "Import email from my other account (POP3)", click "Next"
On the "Enter the mail settings for firstname.lastname@example.org, use values:
Always use a secure connection (SSL) when retrieving mail.
then click "Add Account"
It might ask "Would you also like to be able to send mail as email@example.com?". For this question, if you want mail to leave from your DirectAdmin server, you should select "Yes, I want to be able to send mail as firstname.lastname@example.org". If you use "No", it means email will leave from the gmail servers which may cause your emails to be blocked, if the SPF records don't include gmail's sending IPs. If unsure, just use "Yes" here, click "Next".
The next page is called "Enter information about your other email address". The "Name" field is likely your name, eg: "Bob Smith", but the important part is the "Treat as an alias" checkbox. See this URL to decide what you want to use. If unsure, leave it checked. Click "Next Step".
On the "Send mail through your SMTP server" page, use the following settings:
Secured connection using:
Click "Add Account".
You'll be asked to type in a code. Check your email@example.com email account, if needed use https://www.domain.com/roundcube. Enter the code and press enter.