Page Break on every post

I want to create a page break on every post because of very long content, i put a tag <–nextpage–> in the position i want, but it doesn’t work, how can i do? please help me!

I want to create a page break like this: https://lh5.googleusercontent.com/-2EQAEY7IEKo/Tt_FNniNkjI/AAAAAAAABSg/PhuhlkjuLFo/s653/2011-12-08_025705.jpg

This is my blog: https://vietdesigner.net

Thanks so much ??