Taming one is simple: Stay under him behind his front legs so he cant reach and push you out and start feeding him, he will eventually stop moving. While feeding it tell your friends to stay away or it will start moving again.
It will take some time until he asks for food for The second time. After that its easy, just give him The best food you can to make it faster.
When taming to make it easier stand against a cliff or a good sized rock. I tamed one for my brother and it was a piece of cake with that technique. Also if you find that the place you find the diplodocus isn’t safe just get near it and lead it away. They won’t hurt you they just like to swat you around like they are playing a game of soccer.