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This is a good session!
Here is the condensed version of the number system:
Each number is called a scale degree, and it represents the notes of the major scale. The first note is 1, the second note is 2, etc. So, in the key of C, the notes are:
C=1 D=2 E=3 F=4 G=5 A=6 B=7
These scale degrees will be the bass notes that you play. So, let's take that song "We Wish You A Merry Christmas" and write out the bass notes in the key of C:
Key C LH only
G (5) / We
C (1) / wish you a Merry
F (4) / Christmas, we
D (2) / wish you a Merry
G (5) / Christmas, we
E (3) / wish you a Merry
A (6) / Christmas, and a
D (2) / Happy
G (5) / New
C (1) / Year,
G (5) / good
C (1) / tidings to
B (7) / you and
A (6) / all that you
G (5) / are, good
F (4) / tidings for a
E (3) / Christmas and a
D (2) / Happy
G (5) / New
C (1) / Year.
Now, the scale degrees are a part of a bigger entity, known as the progression. A progression is a group of 2 or more chords being played. Each chord leads or progresses to the next chord. The scale degrees which are your bass note,s are the pattern for the song. Now, all you need is some chords to play in the RH that goes with those bass notes, then you have a completed progression.
Chords are a different story. There are some guidelines, but your ear is the final judge. If you have no idea what chords to play with each bass note, you can play the chords built off each bass note. Chords are built in 3rds, which mean each note is a 3rd a part. Example:
C D E F G A B C
Pick a note, C skip a note, E skip another note, G etc.
So, back to our discussion, the chords built off the scale degrees use only the notes of the major scale and are labeled as follows:
1 = major
2 = minor
3 = minor
4 = major
5 = major
6 = minor
7 = diminished
Using the key of C as an example, the major scale and chords are:
C D E F G A B
1 = C-E-G, E-G-C, G-C-E
2 = D-F-A, F-A-D, A-D-F
3 = E-G-B, G-B-E, B-E-G
4 = F-A-C, A-C-F, C-F-A
5 = G-B-D. B-D-G. D-G-B
6 = A-C-E, C-E-A, E-A-C
7 = B-D-F , D-F-B, F-B-D
So, let's plug these chords in our song and see what happens:
Key C LH/RH
G / G-B-D (5) We
C / G-C-E (1) wish you a Merry
F / A-C-F (4) Christmas, we
D / A-D-F (2) wish you a Merry
G / B-D-G (5) Christmas, we
E / B-E-G (3) wish you a Merry
A / C-E-A (6) Christmas, and a
D / A-D-F (2) Happy
G / G-B-D (5) New
C / G-C-E (1) Year,
G / D-G-B (5) good
C / E-G-C (1) tidings to
B / D-F-B (7) you and
A / C-E-A (6) all that you
G / B-D-G (5) are, good
F / A-C-F (4) tidings for a
E / G-B-E (3) Christmas and a
D / A-D-F (2) Happy
G / G-B-D (5) New
C / G-C-E (1) Year.
See how easy that was? Now, from a musical standpoint, some of those chords sounded good, but some didn't sound so good. So, what you want to do is take the chords that didn't sound so good and pick another chord that may work.
What you can do is mix & match the bass notes with the other scale degree chords and see what u come up with. If you don't want to do that, you can alter the chord by itself. To eliminate a little time, I'll show you the chords I would play to make this sound better:
Key C LH/RH
G / G-B-D (5) We
C / G-C-E (1) wish you a Merry
F / A-C-F (4) Christmas, we
D / A-D-F# (2) wish you a Merry *I sharped the F to from a D major chord
G / B-D-G (5) Christmas, we
E / B-E-G# (3) wish you a Merry *I sharped the G to form an E major chord
A / C-E-A (6) Christmas, and a
D / A-C-F (2) Happy *I played a 4 chord in the RH w/the 2 in the LH
G / G-B-D (5) New
C / G-C-E (1) Year,
G / D-G-B (5) good
C / E-G-C (1) tidings to
B / D-G-B (7) you and *I played a 5 chord in the RH w/the 7 in the LH
A / C-E-A (6) all that you
G / B-D-G (5) are, good
F / A-C-F (4) tidings for a
E / G-C-E (3) Christmas and a *I played a 1 chord in the RH w/the 3 in the LH
D / A-C-F (2) Happy *I played a 4 chord in the RH w/the 2 in the LH
G / G-B-D (5) New
C / G-C-E (1) Year.
Now, compare that version with the versin we had ealier. You should hear a big difference cuz I changed some of the chords in order to make the song as a whole sound & flow better. That's basically how it's done, very simple and time-saving once u get the hang of it.
For more information on scale degrees, progressions, and music theory, visit this link:
http://www.learngospelmusic.com/forums/index.php/topic,42768.0.html 
