How do I make a plugin that shows RELATED POST at the end of each post?

[datcangio]

How do I make a plugin that shows RELATED POST at the end of each he post?
The image would stay at this position:

Thank you very much for your kindly help!

[yariplus]

How would you determine if a post is related?

[datcangio]

@yariplus a post is related to the existing post if they are in the same category. Hope you could give me a solution :)