Go to Apple Developer Program and click the blue Enroll button.

Click on Start Your Enrollment.

In the next screen:

{A} - if you already have a dedicated Apple ID (email address) for your magazine simply login and scroll down to the Apple Developer Program Agreement steps below. 

NOTE: you don't want to join the Developer program with an existing Apple ID (email address) you're using as a consumer. It's best to create a new Apple ID for your magazine.

{B} - if don't have an Apple ID, click on the 'Create yours now' link.

In the next screen, complete the Apple ID form and then click on Continue.

In the next screen, tick to accept Apple Developer Program Agreement and click on Submit.

In the next screen, select relevant Entity Type and click Continue.

Select Option 2: Company / Organisation.

In the next screen, follow the rest of the steps to sign up for the Apple Developer Program.