1) Create a new tag & name it as Scroll Depth tag :

2) Select the track type Event

3) Mention the category name as scroll depth or which ever you prefer.

4) Mention the action name to “Fire a tag based on how far a user has scrolled down a web page.”

5) Mention the label name to {{Scroll Depth Threshold}} A numeric value indicating the scroll depth
that caused the trigger to fire. It will tell you the percentage of scrolling on webpage.

6) Select the Default Variable that is your GA ID

7) Create a trigger and name it as Scroll Depth trigger.

8) Select the trigger type as scroll depth.

9) Select Vertical scroll depth to know the user flow.

10) In the percentage box update 10,20,30… as per your requirement to track the user flow.

11) Enable this trigger on Window load (gtm.load).

12) Finally enable this trigger fires on all pages or selected page where there is a CTA Button.

Finally as i said you have to test in preview mode to confirm the tag is firing, If yes then push to live. Once done then cross verify in Google Analytics real time data under events to know the tag & trigger status.

GTM Preview Mode :

Google Analytics Real time data report :

Drop me a line if I can do anything else for you.