Roll for Shoes doesn't have a concept of "DC" or "passive opposition" unless you invent one. The rules only supply the active variety of opposition: rolling dice!
When a player wants to attempt something and the GM wants to oppose, the GM also rolls. If the player beats the GM's roll, what they want to happen happens. How many dice does the GM roll when challenging the players? will guide you as a GM in working out how many dice to roll.
If you're apprehensive about playing because of this stuff, you might be overthinking it. This is a system where you will probably learn best by doing, not by studying or conemplating. (There are only 7 bullet points in the rules to contemplate, anyway.) RFS is a system built for having light-hearted fun, and it sets things up for people to win regularly. Setting too-low or too-high opposition and making things too easy is something you can correct as you go.
I suggest sticking with the basics at first and seeing how they go, even if you're keen to try out passive opposition. After you've played a bit, you might have the experience necessary to actually craft a passive opposition system you're happy with yourself. At that point, I suggest you look to AnyDice and run simulations on the average results of 1-5d6, and see how many dice it would take for a particular DC to be beaten at least half the time, or even most of the time. That's the basic math behind DC systems: determine how often you want people to succeed at what skill level.