<b:includable id="main"><data:content /></b:includable>
If we keep this code as it is, then related post will appear in in homepage as well as in post page of your post. But if you want to hide it from the homepage, replace above code with:
Once it’s being replaced, click on add Widget button. All the steps from there will remain same just like I have explained in earlier post. Hope you got it right. Cheers!