Hi Alejandro,
Not enough time to write a tutorial this week (though I'd like it to post on my blog), but I can give a very good advice:
Check your xml in Google Search Console of your site and check if duplicates, troubles, so on.
Also Xenu is a good tool to check and define where there are duplicates.
In many cases, dumb written plugins or else generates double canonical for many URLs. That's just one my findings with a dozen managed clients ones.
Have a happy new year,
