An important thing to know in case you’re an unobservant dummy like me: you can only earn 40 trophies in the first tour. Once you do, future wins won’t actually help you. You’ll need to move on to the second tour (wherein you can earn up to 100) or simply play a different mode and win trophies there instead.
Also, losing games loses you trophies. So if you match someone significantly better than you, don’t rematch them if you lose. If you play someone named “guest” it’s probably a bot and relatively easy to beat, so rematch those as much as the game will let you.
You can boost this with a second account on your phone.