9 STEPS
1 The first step is to open the theme settings area of your Shopify theme config and go into the Opt in checkbox settings section.

2 Make sure to enable the feature here.

3 Next, enter checkbox text.
Ex. 'I agree to subscribe to updates from Shoptimized'

4 Next, enter the checkbox link text.
Ex. 'Privacy Policy'

5 Next, enter the link for the checkbox link text.
This is usually the link to your 'Privacy Policy' text.

6 Now, enter checkbox warning text.
This text will show when a customer tries to submit the newsletter form without accepting the terms & conditions.
Ex. 'You must agree with the terms and conditions to Submit!'

7 Next, select the color for warning text.

8 Now click save to save your theme config and preview your changes.

9 Checkbox in Newsletter.
That's it. You're done.

Here's an interactive tutorial for the visual learners
https://www.iorad.com/player/1584280/DemoStore---Opt-In-Checkbox
Due to CAN-SPAM and now GDPR, you need to make sure your customers give consent to receive marketing communications from your company.
Show Checkbox for Terms and Conditions – Toggles opt-in checkbox on or off. We recommend enabling.
Newsletter Checkbox Text– Enter your opt-in statement here. Ex: I agree to subscribe to updates from Shoptimized.
Newsletter Checkbox Link Text – Enter the text for the link you’ll be displaying. Ex: Terms of Service.
Newsletter Checkbox Text Link – Enter the path to your checkbox link. Ex: /pages/terms
Newsletter Checkbox Warning Pop-up text – If a visitor forgets to tick and agree to your terms, a warning will display. Ex: You must agree with the terms and conditions to Submit!