Know that it is easiest to passive tame them, which means walking up to them and feeding them
Get a TON of berries, everytime you feed it, it will go up like 3-5%
Have a high speed, high attack flying dino, (high attack in case you "miss click")
HAVE PATIENCE, diplo's are a stubborn tame, they will push you playfully (hence the flying dino)
MAKE SURE there are no carnivores around, if there are carnies around it will attack it there is a chance of it dying, it will get rid of all your hard work, and most importantly, IT MAY DIE (R.I.P. attempted tame #1 + 2 😢)
Plz note I am not an "high level player" that has a giga, I am lvl 50, (51 at the time I wrote this, almost 52) so I am not that experienced, tho I am sorta, I don't know that much about the game
Everyone saying the Diplo needs a tail whip attack and platform saddle, seriously if you want that get a bronto, the Diplo has its purpose and so does the bronto, making one into the other makes one redundant and totally useless. Brontos are attackers and portable bases, diplos are a hybrid of speed and capacity, no one would complain about a gallimimus that can carry eleven people and still have space to spare for three tonnes of cargo, but they do complain about a Diplo which is essentially that in terms of functionality.
Not as useless as you might think! A high-level spino was terrorizing a nearby river. I took my diplo (Dylan) and while the spino was focused on killing Dylan, I was using my crossbow and rifle to do damage to it. Spino ended up dead, while Dylan was at 2000 out of 3000 health, and I was not injured at all.