You need about 100 Mejoberries. You should probably tame it around a bushy area, so you can keep collecting berries. Do not attack a Diplodocus! Just get close and start feeding it mejoberies. Feed it one mejoberry every time it says to. Diplodocus will knock you around during taming. This is not an attack. It's just playing.
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
Passive tamed a level 12 (+5 levels) with 15 mejoberries. If you can situate yourself under it’s legs, specifically between the two front ones, the diplo will continue to try and bump you back but will be unsuccessful. It will do this move over and over again, and drain its stamina and hunger faster, which leads to a faster tame.