To do this tutorial you need to know your 5ths really well.
1. Practice going up by 5ths from C with your RH, until you get to F# – so C, G, D, A, E, B, F#.
2. Then practice going down by 5ths from C with your LH, until you get to Gb, remembering that you go to Bb after F, not B – so C, F, Bb, Eb, Ab, Db, Gb.
3. Now do 1. again, but this time saying:
no sharps (C),
1 sharp (G),
2 sharps (D),
3 sharps (A),
4 sharps (E),
5 sharps (B),
6 sharps (F#).
4. Now do 2. again, but this time saying:
no flats (C),
1 flat (F),
2 flats (Bb),
3 flats (Eb),
4 flats (Ab),
5 flats (Db),
6 flats (Gb).
5. You can now work out the tonic of any major key signature. Just ask yourself how many sharps of flats there in the key signature are and go up or down by 5ths by that number until you find the tonic.
6. How do you know that the piece isn’t in the relative minor? Look at the last bass note in the piece – if it’s 3 semitones below the answer you found in 5, it’s in the relative minor, but the scale is the same.