There are two standard methods for receiving consent from your recipients. The first, being a sign up form (email your recipients with a link to go fill out that form on your website).
Another alternative is to use a permission pass email. It's imperative that the content you deliver in this permission pass be very similar to normal content delivered on a regular basis. A simple message asking recipients to confirm their interest will likely be perceived (by the recipients' domains) as delivering to a list that has not given permission. As such, delivery of said message may suffer. Providing your normal content as well as your permission pass link will help alleviate this problem.
The idea is to send out an email with a link that reads "yes I consent" (or however you feel you need to work the agreement). That should link that to a page on your site. The URL is irrelevant but it should take recipients to a page that acknowledges that they have now provided consent to be mailed. The important part is to provide positive, clear, feedback explaining that the recipients' required actions are now complete.
Using the ReachMail link tracking system, you can then collect the information from recipients who have clicked on that consent link, creating or appending them into a different sending list. The tracked links section of a ReachMail campaign report provides a convenient link for accomplishing this quickly.
Clicking this list icon will take you to a page which will walk you through adding these "clickers" to a new list, or append them to an existing list in your account.
It is recommended that you wait at 24-48 hours before collecting these "clickers" so as to give an late comers a chance to view your message and provide consent.