This mainly applies to new VDS customers once the server has been set up, but works on a shared server too.

Once the Server has been set up, you will need to log in to create a user on the server to allow the customer access to WHM.

Firstly, you'll need to log in to the server as root and add the primary domain the customer has given us as a new account (there needs to be at least 1 account on the server to allow you to assign the user privileges to access WHM in the future)

Once you have gone through the process of Creating New Account (there's a guide if you aren't sure how to do this), you would need to select the List Accounts option, and modify the account to make it a reseller --- there is a tick option to enable this.

When you have made the user a Reseller, just search for 'Edit Reseller Nameservers and Privileges' and then select the username, followed by 'Submit'.

On the following page, you will see an option to assign certain privileges for the user (these are all the management options the user will have access to once logged in to WHM themselves). As a general rule, you would assign them the following:

Account Information:
List Accounts
View Account Bandwidth Usage

Account Management:
(Un)Suspend
Change Passwords
Create
Edit MX Entries
Terminate
Upgrade/Downgrade

DNS:
Add
Edit
Park
Remove

Packages:
Add/Remove
Edit

Package Privileges:
Prevent Accounts from being created with shell access
Allow the reseller to use all global packages (global packages are any packages without a "_" in them)
Allow Creation of Packages with Addon Domains
Allow Creation of Packages with Parked Domains
Allow Creation of Packages with Shell Access
Allow Creation of Packages with Unlimited Bandwidth
Allow Creation of Packages with Unlimited Diskspace
Allow Creation of Packages with Unlimited Features (ie. unlimited pop accounts)
Allow Creation of Packages with non-default Email Limits

Global Privileges
View Server Information
View Server Status

Once you have assigned these privileges you just need to select to 'Save All Settings'

The last step is to go back to List Accounts and to edit the owner of the reseller account to be the reseller themselves, rather than 'root', as otherwise when they log in to WHM they will not see their own account as one to manage!

If you ever needed to create a new account for a customer on their server, you'd also want to make sure that these accounts were set to be owned by the reseller account, rather than root for the same reason.