All of the below resources provide helpful substantive and methodological guidance to trial monitors.

TrialWatch Trial Monitor Training
This training program teaches the basics of how to be a trial monitor and gathers and describes all of the relevant international standards regarding what constitutes a fair trial.

Office of the High Commissioner for Human Rights, Chapter on Trial Monitoring within their Manual on Human Rights Monitoring

International Commission of Jurists (ICJ), Manual on Trial Monitoring 

Organization for Security and Co-operation in Europe (OSCE), Trial Monitoring: A Reference Manual for Practitioners

Front Line Defenders, Trial Observation Handbook for Human Rights Defenders