Do you follow all these leagues? How do you predict the leagues you don't watch.
I don't follow them all in the sense of watching them etc. But generally you know the best teams in leagues, who plays for them etc. They also post odds for most games that give you an idea of the favourites etc. Even if you don't follow the leagues, it is worth playing. It is free (though I pay an optional £2 a month to remove ads) and just for fun.
After that, it's a lot about luck. I have predicted upsets here and there and it works out for me sometimes. But generally it's about sense - clearly it's very silly to predict Southampton to beat Man City in Manchester. Whereas something like West Ham vs Palace is more of a toss up.
Many people predict 2-1 scorelines too for those tight games - I would bet that is the most common winning margin (either way). Generally because one team equalises then the other goes on to score a winner.