Question For An Advanced Guru Re Link Ups?

I know what I want to but don't understand the HOW behind it. Here's where I am at:
I have an ebook site which I sell using clickbank at the check out.
I have a Membership site, same niche, using Amember and paypal (in case you need to know that part).
When people buy my ebook and give me credit card details I want to give them a free 30 day trial of my Membership site. Armand Morin says to do this but I don't know the "how" behind it (sorry I am a bit thick sometimes).
Now, do we just say "ha, gotya, we're goping to bill you again in 30 days?" I kind of think people might be a bit annoyed. Please explain how we let them know gently that they have agreed to take the 30 day trial, and WHERE does this happen? Is it when they go to download product we tell them? Surely we need to give them box to tick (yes, I want the 30 day free trial)?
do we put in another sandwich page after they agree to buy (and after my upsell page), asking them to agree to the trial?
I do hope someone understands me. I just don't understand where to put stuff to make it ethical. I understand I tell them upfront that they ahve a chance to cancel but need guidance. PLZ help... many thanks for all answers I do appreciate it as I really want to get it right and male my millions (HA)! If anyone has an example page they could point me to, they would be most outstanding...