Go to editing article and under text editor find the block Related articles
Select article in the list to which you want to add a link. Data (URL, text and photo) will be filled out automatically from the selected articles. You can correct them at your discretion.
In order to add one more post – click Add new post button.
In order to add arbitrary link – just paste it in URL field, specify your title for this link in Text field and set photo, uploading and selecting it via Upload image button.
At the very bottom of block you will find block preview (it will look approximately like this on your site)