2.3Now build D major

Use exactly the same pattern:

D E F♯ G A B C♯ D

Why?

D → E = whole E → F♯ = whole F♯ → G = half G → A = whole A → B = whole B → C♯ = whole C♯ → D = half

So D major has:

F♯ and C♯

This gives us the first little pattern:

C major: no sharps

G major: F♯

D major: F♯ C♯

A major: F♯ C♯ G♯

E major: F♯ C♯ G♯ D♯

Do you see where this is going?

Each time we move up a fifth, we add one sharp.