Choose a key in which to practice your scale. It makes little difference which key you choose, since scales in every key generate the same patterns on the fretboard. Let's assume you've picked the scale of G major, the notes of which are G, A, B, C, D, E, and F#.
Place your index finger on the sixth string, just behind the third fret. Pluck the string downward with the pick. This note is G, and it is the first note of the G major scale, also referred to as the "tonic" note.
Place your middle finger on the sixth string, just behind the fifth fret and pluck the string upward. Place your pinky finger on the sixth string, just behind the seventh fret and pluck the string downward. These notes are A and B, respectively, and they are the second and third notes, or "degrees," of the G major scale.
Repeat the previous 2 steps a few times, playing from G up to B and back down. Alternate upward and downward strokes of the pick.
Play the third, fifth, and seventh frets on the fifth string. These are the notes C, D and E, which are the fourth, fifth and sixth scale degrees of G major, respectively. Notice how these notes fall on the same frets as scale degrees on through three, just on the fifth string instead of the sixth.
Play the fourth and fifth frets on the fourth string, F# and G respectively, to complete the scale. Throw in the seventh fret, A again, which can be referred to as the second or the ninth. Notice how the pattern is the same, except you're moving your index finger up one fret.
Play the fourth, fifth, and seventh frets on the third string, B, C and D respectively. Again, notice the pattern is the same as the fourth string, with the index finger shifted forward one fret relative to where you started.
Play the fifth, seventh with your ring finger, and eighth frets on the second string, E, F# and G respectively. There are two things of note here: The entire pattern shifts up one fret because the tuning between the second and third strings. The pattern also has changed; the half-step now occurs between your ring finger and pinky instead of between your index and middle fingers.
Play the fifth, seventh and eighth frets on the first string, A, B and C respectively. Notice the pattern is identical to that of the second string. There now are three repeating fretboard patterns: Index, middle, and pinky fingers separated by a whole step each, half-step between index and middle finger, and half-step between ring and pinky fingers.
Play pattern one, shift up a string, play pattern one again. Shift up a string, play pattern two, shift up a string, play pattern two again. Shift up a string, play pattern three, shift up a string, play pattern 3 again. Shift up a string, play pattern one. This pattern gives you a major scale anywhere on the fretboard. Move everything up one fret when shifting from the third to second strings.
Set your metronome to a tempo slow enough for you to play perfectly these patterns. Practice with the metronome, gradually moving the tempo up as you get more comfortable. Alternate upward and downward pick strokes as much as possible.