How to create a custom field formatter?

Question

I look for create custom field image widget with image styles for drupal 8.

I need something like that:

// Here for example with a "Large preset"
<div class="wrap1"> 
    <div class="item"><img src="1.jpg"></div>
    <div class="item"><img src="2.jpg"></div>
    <div class="item"><img src="3.jpg"></div>
</div>

// Here  same images but with a "Thumbnail preset" 
<div class="wrap2">  
    <div class="item"><img src="1.jpg"></div>
    <div class="item"><img src="2.jpg"></div>
    <div class="item"><img src="3.jpg"></div>
</div>

I read this https://www.drupal.org/node/2620966 but still don’t understand how to applicate it for my objective. P.S. I mean to make it in a custom module.

I don’t have any idea how to make it. I’ll very thankful for any help!