Overall groups standings

[FCCK]

Hi,

I have been searching in the forum for a while and haven't found the information I need, I hope you are able to help.

I have created a competition thatbis divided into groups. After the grouo stages there are playoffs. The qualification to the playoff won't be determined by a specific number of places per group, but the best n teams overall. 

A similar system is employed when determining the best second place (euro qualifications) or the best third places (euro 2016). The Libertadores Cup ranking for playoffs also uses an overall table.

I would check those competitions but I'm having a hard time trying to convert them to advanced rules.

Thanks in advance,