2.25Build chords by number

In G major:

I = G

ii = Am

iii = Bm

IV = C

V = D

vi = Em

vii° = F♯°

Now randomly play:

I → vi → ii → V → I

That gives:

G → Em → Am → D → G

Then do the same pattern in C:

C → Am → Dm → G → C

Same chord functions.

You are starting to build a transferable harmonic vocabulary.