how to make trainer on online games?

[JMC17]

It is possible to memory hack multiplayer only when the game lacks the protection against it.

Neverwinter Nights 2 and Titan Quest are two games that I know you can hack on multiplayer.

But most games, you cannot.
If you want to hack on multiplayer, I suggest you read about WPE Pro and packet sniffing/filtering. Although that's totally illegal.

I noticed you've already posted for another online game, listen to everyone, forget multiplayer hacking.
In the end it just ruins the fun for other skilled player who loves the game.