What did the founders do to make sure the military did not get to powerful?

They did many things but two are most famous. One is that all wars or similar activities may only be started officially by the congress and not by the president who is the chief of military which means that the president would not be able to take the country to random wars that suited their benefits. This was somewhat exploited historically but mostly sticks

Another thing they did was add the second amendment. The idea of right to bear arms and organize in paramilitary organizations was made in order to limit the power of the government and its military. It was based around the idea that if the government becomes too powerful and controls its people through the power of the military, then the people have the right to fight back with their own weapons. In theory, an average man can procure all the same pieces of equipment and weapons as a professional soldier if such cases are needed.