There are two main paths:
you have no clue what to do, and you have been named legatus for political or nepotistic reasons. In this case, you do what any unqualified commander should do in the field: lean hard on your sub-officers, in this case the centurions of the first rank and perhaps the tribunes of the soldiers, and make firm but informed decisions based on their input. It is perfectly acceptable to consult them, either formally or informally. Thus when Caesar's legates Cotta and Sabinus are trying to make a difficult decision in the field, the tribunes and centurions are there with them and their voices are heard (Caes. BG 5.28).
you already have some good military experience because you served as a tribune of the soldiers when you were a young lad, then got further experience as a quaestor attached to a proconsular governor somewhere dangerous, like Syria. Bonus points if you had also been praetor previously (as was typically the case).
In other words: there is no "military academy" where one would go to learn to be a commander, no "officer training school." The training was informal, in the form of advice and adages passed down from the older generation to the younger, either informally or through systems of apprenticeship like the relationship which could develop between tribune or quaestor and commander. The position of the tribune(s) of the soldiers was perfect for this informal training in the Late Republic: you had no formal command role, but could hang around those who did and learn by watching. You could be present where the decisions were being made, in other words, and brush shoulders with the centurions, who were fonts of knowledge on how the army actually worked: how the castra is made, what the soldiers love and hate, etc.