After a lawyer receives a case, whether it is a criminal case or a civil case, he must first understand the general case from the client. If the client's defense request is acceptable, the client (or the party concerned) needs to sign a power of attorney to inform the client how much legal fees need to be paid according to the relevant regulations of the law firm. Then enter the investigation and evidence collection before the trial, meet with relevant personnel (parties and witnesses) and obtain relevant evidence (witnesses and testimonies), prepare defense statements and participate in the trial.
What this friend said above: "Contact the police, procuratorate and court according to different stages" is not completely correct. In criminal cases, the police handle cases independently, and only after the investigation is completed will they be handed over to the procuratorate for public prosecution. Lawyers can consult the relevant police records when obtaining evidence. The court only tried the case according to the indictment provided by the prosecution (procuratorate). Before the trial, lawyers generally do not contact the court.