It costs 3 resources to try to develop a technique. The attempt to develop a technique can only take place when you have an armory and a hero (excluding confessors) on that territory with the armory. If you do not have an armory and a commander, prince, or general on a territory with an armory, you have no choice but to skip the Development Stage.
If you have a hero’s army in a territory with an armory, you can try and develop a technique. Each time you try to develop, it will cost you 3 resources. To try and develop a technique, roll 1 confessor die. If you roll a cross or 1 of the 2 sides with shields, you have failed to develop a technique. If you roll 1 of the sides with pikes, you have successfully developed a technique (see pic below).
To see what technique you will receive for your army, roll a confessor die again. This time, if you get 1 of the 3 sides with the pike/pikes, you get the next offensive technique on the game board (starting with 1 and working up to 3). If you get the shield/shields or cross, you get the next tactical technique on the game board (starting from 1 and working up to 3). There are 6 total techniques (see pic below) that you can develop in the game (3 offensive and 3 tactical). The more techniques that are developed, the better chance you have at getting a great technique for your army. If all the offensive markers are allocated to the armies, then you are guaranteed to get 1 of the tactical techniques and vice versa. When the techniques are gone, they are gone.
Once you know which technique your army has successfully developed, take the corresponding technique marker on the game board and place it on the army mat of the hero that is in the territory with the armory. That army now will have that technique for as long as that hero survives. Once the hero with their army is defeated on the field of battle, that technique marker will go back on the game board on the Technique Chart. It is now available once again for a different army to develop at an armory.