Step One: Eliminate or Assimilate the Opposition
The authority of the Dark Lord must be absolute. It is absolutely critical for any dictator or warlord to either destroy or convert any potential rivals or rebels to their cause. Your best real world parallel would be Genghis Khan, who united the Steppe Tribes through violence, marriage alliances, and shrewd negotiations. He eliminated rivals that couldn’t be persuaded with extreme brutality, and richly rewarded the competent and loyal. Your Dark Lord killed rival chiefs and champions in combat, rewarded loyalists with status and wealth, and was constantly acting as a mediator and negotiator between aggrieved factions. Act like a king and soon enough you are.
Step Two: Slaves and Serfs
An army marches on its stomach, and professional soldiers can’t properly balance their military duties with intensive agriculture. This is why Sparta was a slave society in which over 80% of the population were slaves (helots). Knights and Samurai were similarly supported by the feudal system in which most of the population was working full time in agriculture. So your Dark Lord needs to ensure that food production is being handled and that the professional warriors can be training and preparing for war full time. The slaves/serfs could be the remains of Orc factions that refused to join and were thus turned into slaves that the loyalist factions then had ownership of. Later on they would be prisoners of war and populations from polities that surrendered to the Dark Lord’s might. Social hierarchal are critical for maintaining control of society and bending it to militaristic ends.
Step Three: Effective Training
The strength of Sparta and Rome was in how well disciplined it’s soldiers were. The Dark Lord must ensure that the professional warriors are a cohesive and disciplined force that responds to orders immediately, doesn’t undercut the chain of command, and has an officer corps that understands strategy and tactics. These officers should be selected on the basis of skill, just as Genghis Khan did.
Step Four: Unifying Ideology
People (and presumably orcs) never fight for no reason, and they most definitely don’t follow someone else for no reason. There must be a motivation beyond pure material gain. Whether it is a culture of extreme militarism like Sparta, a chauvinist spirit like Rome, or religious fanaticism like the caliphates, there’s got to be something that justifies the new order and continual military expansion.
Don’t Forget Logistics and Communications!
Amateurs study tactics, masters study logistics. You can have the best warriors on the planet, but if they starve before they can win the war or can’t coordinate then they’re not going very far. The Dark Lord needs to heavily invest in supply trains, foraging protocols for the army, and food preservation. Dried and salted meat would likely be a good choice for orcs at a medieval level.
For communications the Dark Lord could have a pony express system of sorts with riders (if they have any sort of Calvary) or message runners, homing pigeons, flags, kites, smoke signals, fuck it, even your wyverns carrying child orcs carrying a scroll.