It's really simple. My newsletter CTA looks like this:

https://cdn-images-1.medium.com/max/1520/1*pDNISBRJFawWMkNX7...

Just photoshop together a pretty image inviting people to subscribe.

My free guide cta looks like this:

https://cdn-images-1.medium.com/max/1520/1*MrvefAFP7GtyAtPZY...

Put together a short pdf in your area of expertise, and offer it to email subscribers.

Sometimes I simply send people to a landing page that looks like this:

https://startuplab.io/free-guide

Keep it simple, minimal information, promise value, address potential objections, show a signup form and a big subscribe button.

Here's some good advice on putting together a simple landing page:

https://www.youtube.com/watch?v=GVRf3MeFKm0

If you're interested in email marketing in general, I highly recommend Alex Becker's courses. I don't know if HN audience will like him, but I've learned A LOT from this guy and I love his tutorials. You can find his introductory course here:

http://alexbecker.org/membership-area/

Feel free to email me if you need more advice([email protected]).