Simply select the titles you'll like to receive, each title you select will have an issue number that your subcription will start from. If you missed an issue, find and add that issue to your current order with the subscription. Make sure you select Parcel Post at checkout for your 1st initial order.
Once your subscription is placed, you will receive a note in your account under subscription with a list of current release dates of the titles you selected. This is your guide to when your subscription with be posted.
On your 2nd subscription order and onward you will be charged at the end of all titles/postage once all titles are released.
Now that you have an active subscription, any order you place afterward can be combined and sent with your subcription. Just make sure to select "Send with your monthly subscription" at checkout.