What advice can you give for someone who is pro per on a criminal case?

It is important to ask for a second opinion from a professional legal adviser regarding your decision to exercise propria persona. While exercising your right to represent yourself may be fine in a civil case, a criminal case is more complex, depending on the nature of the crime. In a typical criminal case, a criminal defense attorney is assisted by a team of associates, paralegals, expert witnesses, and even private investigators.
If you do decide to go forth with pro per, it is advisable for you to gather all relevant information regarding the case—for instance, witness testimonies and physical and/or digital evidence (i.e., digital photos, videos, and other media). However, regarding the former, you may be liable for witness tampering if you do decide to contact witnesses for a testimony. That is why it is advisable that you employ an experienced attorney, or at least consult with one.
Additionally, you must be methodical in how you gather testimonies. For example, you should have blank affidavits at hand to officially record witness testimonies. It is preferable that statements are written in the presence of a notary public so that the affidavit can be notarized.
Under no circumstance should you try to conduct physical surveillance, because only state-licensed private investigators can do so legally. Likewise, make sure you are aware of federal and state anti-wiretapping laws.
You must also learn legal definitions and court procedures. Learning state laws, federal laws, criminal procedures, and legal definitions is one of the essential steps in preparation for a trial.