These eloquent and heroic speeches given on the battlefield most likely didn't happen at all, at least not in the way we usually see them portrayed. There's a very old tradition of including them in accounts of battles – medieval writers modeled their accounts of battles on ancient sources, which almost always included these sorts of speeches (so bear with me if I bring in ancient sources to help answer your question). But these orations we see in written texts were almost definitely composed after the fact.

There's a few reasons to believe this:

Feasibility - There are tons of practical problems with these speeches as we usually think of them. Imagine trying to address a crowd of several thousand people without amplification, sometimes having to talk over the noise of armor, horses, wind, and the other army on the battlefield. Remember that these armies would have been spread out over a mile or more. It's a romantic image, but there's simply no way that a general would be able to address even a solid portion of the soldiers at once. Even repeating the message at different intervals would require around at least 20 repetitions for a large army, not even considering the lines of soldiers farther back on the field.

Enemy Speeches - You’ll often find that a single source will record the speeches of both armies, in complete detail. There’s obviously no way, for instance, for Tacitus to know what Calgacus the British chieftain said to his army (presumably in some Celtic language) before battle against the Romans. Not to mention that ancient writers often liked to embed their own politics and social criticism into the speeches of the enemy – Tacitus, for instance, makes sure that all of his barbarian generals criticize the decline of traditional Roman virtue and their hatred of liberty, the exact issues he was worried about during the early Principate. These speeches are clearly rhetorical inventions by Tacitus, so it’s not too much of stretch to say that the same thing is going on for the “home team” speeches.

Absence in Rhetorical Literature - This one actually relates very directly to your question. Rhetoric and oratory skills were particularly central in ancient Greek society, so we see plenty of manuals and handbooks on how to craft a good political speech. These handbooks included examples of famous speeches as well as theoretical models for aspiring orators to follow. If grand speeches were indeed something that every good general did before battle, then we would expect to find some shred of evidence for it in the form of a handbook or a theoretical model. But while there’s plenty of examples of these speeches given in historical accounts, there’s not one example of any to be found in the rhetorical literature, for the Greeks, Romans, or medieval societies. This absence is very telling, and really suggests that generals did not ever deliver the grandiloquent speeches we read about.

It would be a mistake, at this point, to assume that no kind of oration ever occurred at any point before ancient or medieval battles. What seems most likely is that a number of more mundane things took place. Generals might have addressed portions of their troops in camp, before setting out for battle. Generals could also address their subordinate commanders before setting out from camp, who might then take condensed versions of the speech to their respective regiments. On the battlefield, we have plenty of examples of generals riding across their length of their lines before engaging, and it’s quite likely that a few ad-libbed remarks were given as the king passed. Mogens Herman Hansen makes the argument that many of the pre-battle speeches we read about in the literature (particularly for the Middle Ages) could have been pieced together from snippets of this dialogue, exaggerated and given some rhetorical flourish later on for effect. The medieval sources are particularly useful in this regard, since we often have several different accounts of important battles, of varying proximity to the event. Generally, the closer a chronicler was to the actual battle, the less grandiose the pre-battle speeches were.

So, to come back to your question – there are really no manuals or guides from the Middle Ages on how to make speeches before battle. There’s very little reason to believe that eloquent speeches were ever given on the battlefield at all. We can reasonably speculate that some kind of exhortation was given by the lead general or his subordinates, but this would have happened in camp, before setting out to battle. On the battlefield itself, any exhortations from the general would have been limited to some scattered remarks to the front line.

Sources:

Mogens Herman Hansen, “The Battle Exhortation in Ancient Historiography. Fact or Fiction?,” Historia: Zeitschrift Für Alte Geschichte 42, no. 2 (January 1, 1993)

C. T. H. R. Ehrhardt, “Speeches before Battle?,” Historia: Zeitschrift Für Alte Geschichte 44, no. 1 (March 1, 1995)

Also, how did they reach all the soldiers on the battlefield? One man can't possible reach thousands of warriors.