Make my own shipping module

Hello,
I'd like to make my own shipping module, as I can't find one that has the shipping fees I want.
But I don't know how I should proceed..
I would like to make it very simple, like :

"if($this->poids<=0.5) return 14.5 ;
else if($this->poids>0.5 && $this->poids<=1) return 18.50;
else if($this->poids>1 && $this->poids<=1.5) return 23.50;
else if($this->poids>1.5 && $this->poids<=2) return 28.50;

......"

This kind of function.
Could somebody tell me how to do it ?
Thank you for your help !

What exactly are you trying to achieve? What are your criteria?

If you can explain that to us then there may even be a ready made solution.

Thank you for your answer. Actually, for each order we will first have the product shipped inside Korea to our buyer, and then to our country. That's why we would to make our own shipping price..