#1
well ive been playing guitar for three years, im quite good at it but i now realise to progress that i need to learn a lot more music theory because i suck at basically wrriting full songs. i have the ability to play just not the mindset for what i want to do completely.

so i got this book and it uses the natural scale over and over in different techniques in all 7 modes for 90 pages and i was wondering that if i want to memorize all the notes across the fretboard is it good to only play the natural scale for a good while?
also should i say the notes out loud to really sink them in?

#3
I think i was in the same situation as you a couple of months ago.

What helped me was (in this order)

1. memorizing the notes on the E string
2. memorizing the Ionian mode through one octave on the E string
(E -----------0-2-4-5-7-9-11-12----) play nursery rhymes on that string (mary had a
little lamb, row your boat, twinkle twinkle)
3. Draw the "scale shapes" out in chord diagrams with diamonds for the roots and dots for the notes like i have done for the Ionian (I used X's for the Roots and O's for the other notes)
Ionian
E A D G B e
-----------------
| O O O | O
-----------------
X O | | O X
-----------------
| | O O | |
-----------------
O O X O O O
-----------------
| | | | | |
-----------------

4. Memorize the Ionian shape i gave you. If you want to play A Ionian, put the X's on the E strings on the 5th fret(A notes). as you play it say aloud Root, 2nd, 3rd, 4th, 5th, 6th, 7th, Root....

5. Think of the Ionian(Major) as a reference point. Because all music theory uses it as one anyway. It has root, major second, major third, perfect fourth, perfect fifth, major sixth, and a major seventh.

Ionian I- Major Second, Major Third, Perfect Fourth, Perfect Fifth, Major Sixth, Major Seventh

Change the following to play the

Dorian II- Minor Third, Minor Seventh
Phrygian III- Minor Second, Minor Third, Minor Sixth, Minor Seventh
Lydian IV- Augmented Fourth
Mixolydian V- Minor Seventh
Aeolian VI- Minor Third, Minor Sixth, Minor Seventh
Locrian VII- Minor Second, Minor Third, Diminished Fifth, Minor Sixth, Minor Seventh

to make a Major interval to a Minor Interval, flatten it one half step(one fret lower)
to make a perfect interval to an augemented interval, sharpen to one half step(one fret higher)
to make a perfect interval to a diminished interval, flatten it one half step
to make a major interval a diminished interval flatten it on whole step(two frets lower)

6. Knowing the intervals makes it easier to memorize the scale shapes. Just think like this, "I know the Ionian inside and out. So to play the Phrygian, I just flatten the 2nd, 3rd, 6th, and 7th intervals. That's easy! I can also reference that scale shape I drew earlier too!"

7. Now to master the fret board. Play G Ionian. Play A Dorian. Play B Phrygian. Play C Lydian. Play D Mixolydian. Play E Aeolian. Play F# Locrian. You just played every note in the G Major scale over the entire fretboard. All of those scales are in the key of G Major. Mixolydian is the the Vth mode. D is the Fifth of the G scale. So therefore D Mixolydian is in the key of G major.