Paste a chart, say the key, and read it in numbers. Or the other way round.
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That is the whole thing. On a normal chart, moving a song for somebody's voice makes every letter on the page wrong. On a number chart, 1 4 5 is still 1 4 5 — the band works out the letters on the way to the microphone. It is why a session player would rather be handed numbers.
In G, that is G A B C D E F♯ — so G is 1, C is 4 and D is 5. A minor chord gets a dash after the number, which is why Am in G is 2-. Anything from outside the key carries a flat or a sharp in front: the F in a G song is ♭7. Roman numerals say the same thing with the quality in the case.
Chas Williams' The Nashville Number System — the book the whole thing is written down in — builds every chart on the major scale, so a song in A minor is counted against C and its home chord is 6-. Plenty of players who mostly read minor material count from the minor tonic instead and read the same song as 1- ♭7 ♭6 ♭3. Both are on this page and neither is the advanced one. Roman numerals don't have the argument: they always count a minor key from its own tonic.
Eight keys, the ones bands actually call. Any other key is in the box above.