I run it with a very simple, very open technique.
Suppose the party is low on resources, HP, and has an unconscious character. They are surrounded by orcs and looking at probably defeat. The orc captain will tell them to lay down their weapons or they will be torn to shreds. His intimidate check came up as 16+3. I will then tell the party "He rolled a 19 for intimidate. Act as you will, but be aware that I might use your response as a guide for determining the DC for frightening future enemies into surrender. Tell me what motivates your decision."
Some players will use Wisdom saves or character traits as justify why they keep fighting. Others might accept that they are frightened in this instance and surrender. Others still might declare that they will continue fighting because there was no mechanical consequence. I allow the PCs to choose freely. Doing this several times will eventually give me a spectrum of results to consider.
Three sessions later when they try to intimidate their enemies into surrendering and roll a 17 I will remember how the party responded here. If the players have decided that an intimidate roll of 19 will not cow even the most outmaneuvered, outnumbered, and wounded of groups, then the rest of the world will be brought in line with that. If they decided that making a Wisdom save with a DC equal to the intimidate check is appropriate, then that's how NPCs across the world will be treated.