We will assume you own "yourdomain.com" and "yourdomain.net".
You would like "yourdomain.net" to point to "yourdomain.com".

Here's how you do that in your WHM

(1). Access your WHM.
(2). Click the "Point or Park a Domain" link as below



(3). Select the domain where you would like the pointer to be parked [on the left]
Type in the domain name you are parking [on the right]
Then click the "Do it" button, as below



(4). That should give you the following confirmation screen



Make sure that your DNS information for your parked domain is the same as for your main domain.