There are a number of processes and procedures that result in a fair trial.
The first of these is the simple fact that people get a trial and that the verdict is given by a jury. This makes it much harder for the government to “railroad” someone without good evidence that they are guilty.
Another procedure is the one in which the person must be found guilty “beyond a reasonable doubt.” This means that the government must really present a very solid and even airtight case before a person can be convicted.
Another process is the process of jury selection. Here, the defendant’s lawyer gets to help select the jury. This means that the government cannot simply put a jury in place that is guaranteed to convict the defendant.
Another procedure is the fact that the defendant has a lawyer in the first place. In the US, every defendant in a criminal case must be allowed to have a lawyer. This means that a person at least has some hope of being well-represented in the complex proceedings of a legal trial.
Another procedure has to do with how the police and the prosecutors can obtain evidence against the defendant. They cannot force the defendant to confess. They cannot illegally search the defendant as a way of finding evidence. They have to follow procedures that respect the rights of the defendant/suspect.
All of these are procedures and processes that help to ensure that a defendant will get a fair trial.