You need cards numbered one through twenty. One of each, i.e. no duplicates.

You treat cards equal or lower to your initiative range as if they were twenty higher. E.g. you have noted +2 so ones are twenty-ones for you and twos are twenty-twos.

If you have a negative initiative bonus, like -2 or in the above table, instead treat the highest cards as if they were twenty lower. E.g. you have an initiative score of -2, so 19 is -1 and 20 is 0.

Two ways to use it

The Way of Order

Everyone has a face-up card. They act in order of those numbers, highest to lowest, flipping their card face down after they have acted.

The Way of Chaos

Everyone has a face-down card. They act in a chaotic, natural, “what makes sense in the moment” order. They flip their cards up as they act. If there’s a collision (you both reach for the gun, or, you’re both trying to harm each other) the highest number acts first.

New Deal

You hand your card in, it gets shuffled, you get a new card. At the end of combat (if so, everyone just flips their card between round) or at the end of every round (if so, everyone gets a new card every round), that’s up to the group.

Other deck sizes

Every card needs to be unique and the numbers need to be in sequence.
So 1–60 works, as does 1–100, 1–20, whatever.

To calculate the initiative ranges (ask a math nerd you know to do this), first divide the number of cards in the deck by twenty. That number you get — for decks like 1–9, 0–9, 1–10, 0–10 etc it’s pretty much one half — is what you multiply with the initiative bonus to find the upper range. So for a 1–20 deck, that’s the easiest, you just use your initiative bonus as the range. +4 is 1–4 etc. For a 1–100 deck you multiply it with 5. +4 is 1-20. The number you add is a nice rounded-up version of the highest card in the deck. So with the hundred card deck example if your range is 1–20, you treat those carss as if they were 101–121.

For weird decks like a 3–35 deck: the number of cards is 33 so to calculate the ranges it’s 33/20 and then add two to each number (since the lowest card in the deck is two higher than one). So +4 means treating 3–8 as if they were 103–108.

I actually like the smaller decks better, like 1–10, 1–12 etc or 1–9 also works. If you have more combatants than there are cards, consider grouping them or letting all henches and horde monsters have 5.5 or something (using buttons or markers to keep track of their actions). Or make swarms out of them.