in C major the Mixolydian mode gives G A B C D E F.
If you flatten the 7th note in G Major gives G A B C D E F.

So for a G Dominant 7 chord (GBDF) you can play the C Major scale in the bar.

in lesson one, the focus is mainly to get you to improvise in the scale of the tune ie the notes in G major are used. With a chord having 4 notes means you have the option of starting at 4 different points in the G Major scale or 4 different modes to be precise.

scales are like different languages, so if you talk in the same language then the everything matches up, so the improvisation using G major notes for a Tune set in G Major is a safe bet, compared to using notes from a different Major Scale.

So for a very basic improvisation, start off by learning to improvise just using notes in the Scale of the tune, before trying more difficult stuff, which in this beginning stage you need to practice all the modes in a major scale on its own, so you can do a simple Chord match up, before doing any improvisation with the backing track loop.

So William for the 2-5-1 music sheet, create a loop to repeatedly play back the three chords in the 2-5-1 order
ex G-C-F if the song is in F major.

Then for each chord
Gminor7 (G Bb D F), Cdominant7 (C E G Bb), Fmajor7 (F A C E)

bar 1 play the mode starting with G (A Bb C D E F G)
bar 2 play the mode staring with C (D E F G A Bb C)
bar 3 play the mode starting with F (G A Bb C D E F)
bar 4 play the mode starting Bb (C D E F G A Bb)
bar 5 play the mode starting with E
bar 6 play the mode starting with A
bar 7 D ///
bar 8 G ///
bar 9 C ///
bar 10 F ///
bar 11 Bb ///
bar 12 E ///