Post pagination

I have been searching for several hours on how to solve this problem but I have not been able to find a clear explanation on how to do it:

1) So… let say that there are 60 3osts on the first page and it is getting longer; so long that it needs scrolling and scrolling.

2) And let’s say that I want to have just 15 posts the first page and 15 posts on the next two pages.

How can I achieve that?

I have seen explanation saying to insert <!––nextpage––> but what I have not been able to understand is ‘where’ should I insert that code? Some posts say to put it in the editor but ‘where’ the editor. I do not want to split the post like a ‘read more’ split. I want to split the posts in pages and have navigation numbers on the bottom of the blog like Pages 1, 2, 3,4…

Thank you.