There usually aren't contested rolls. Instead of "PC rolls to sneak and tries to do better than the NPC roll to spot", it's just "PC rolls to see if they can sneak past a guard".
It's up to you if you want to add extra difficulty modifiers to the PC roll. Outside of combat, I usually just use 5 difficulty levels:
- Easy enough that you can do it without a roll (because you have suitable skills, equipment, or circumstances in your favor).
- Easy, but not guaranteed, so roll with advantage.
- Normal
- Hard, so roll with disadvantage.
- Too hard. "You can't get there from here" so try something else.
For sneaking specifically, some people might ask "Shouldn't it be harder to sneak up on soldiers who are on guard duty than just a normal person?" My reply is sneaking past guards is the normal use case for sneaking, so it's just a normal roll with advantage or disadvantage for anything really unusual.
Disadvantages would be things such as a monster with unusually keen senses or guards after an alarm has been raised so they're actively looking for you. I don't worry too much about the difference in difficulty of sneaking up on NPCs with Wisdom 11 vs 14. If you want difficulty ratings, you could just use NPC Ability -10 as the difficulty (so you would need a successful roll higher than 4 to sneak past the Guard with a 14).
For advantages, I like to leave it up to the players to think of ways to tip things in their favor. Some things I've seen include timing the guard patrols to pick the best moment to run for the fence instead of just trying to jump from shadow to shadow like a ninja, throwing meat as a distraction for guard animals, and using a slingshot to throw a pebble to make a noise for guards to investigate "over there".