The new General Data Protection Regulation (GDPR) requires you to have consent for all of your subscribers. Not only the new one but also for your existing once. To get the consent of your subscribers in Mailster you should send a dedicate campaign to all your subscribers and ask the to give you permission to send further emails.
Implement a terms of service checkbox.
Create a new custom field to store the state of the permission (unchecked by default). You can follow this guide on how to do that.
Create a Landing page
Next step is to create a simple landing page where you thank users for their engagement.
Setup your campaign
This kind of campaign should only have one purpose: Get the consent from your users for future emails. So keep the content to a minimum.
Insert a call to action button with a link to your landing page from above.
Configure the helper plugin
You can get the free helper plugin here. After plugin activation you find a new tab on the newsletter settings page:
Enter the Campaign ID, the Link from your landing page and the custom field you like to update.
Start sending your campaign.
Once a subscriber click the link in your campaing the custom field will change and you get recorded activity on the susbcriebrs detail page:
Add a condition so your custom field value must be true:
Please note this is not a legal paper and should just help you to get started. Contact your lawyer or more info on the GDPR and your local laws.