If you are just starting off, you are probably making things too difficult for yourself. Before you start dealing with alterations and extensions, just start with the basics! There are five main chord qualities that are very common in jazz...
Here are the examples using F as the root of the chord.
Major 7 (F, A, C, E)
Dominant 7 (F, A, C, Eb)
Minor 7 (F, Ab, C, Eb)
Minor 7b5 (Half Diminished if you prefer) (F, Ab, Cb(B), Eb)
Diminished 7(F, Ab, Cb(B), Ebb (D))
Try to learn those basic chord types and inversions on at least three different string sets.
Start with 6th, 4th, 3rd, 2nd strings, then 5th, 4th, 3rd, 2nd strings, then 4th, 3rd, 2nd, and 1st strings.
Once you get all of that under your fingers, you will have all of the fundamentals that you need to play through most jazz standards.
After that, you STILL don't need to worry about alterations, and extensions yet! There is an easier way to approach extensions! What you can do is substitute chords diatonically to give you different extensions. For instance I, iii, and vi chords can be substituted for each other. What this means is that if you have a Fmaj7, you can count up 3 from F and play an Amin7, or count up 6 and play a Dmin7. Look at the notes...
Fmaj7: F A C E
Amin7: A C E G (Fmaj9 sound)
Dmin7: D F A C (F 6 sound)
See, in this instance you don't need to learn a whole new set of voicings for major9 or 6 chords, you can just get more mileage out of the voicings you already know! The same thing works for subbing the ii and IV chord. In the key of F that is Gmin7 and Bbmaj7:
Gmin7: G Bb D F
Bbmaj7: Bb D F A (Gmin9 sound)
Cool right? The same thing works between the V and the vii, in the key of F this is C7 and E-7b5.
C7: C E G Bb
E-7b5:E G Bb D (C9 sound)
If you want to get an altered dominant sound you would swap a diminished 7th chord for the min7b5 chord...
C7: C E G Bb
Edim7: E G Bb Db (C7b9 sound)
Once you have all of this down you will be able to play Maj7, Maj9, and 6th chords, Min7 and Min9, dom7, dom9, and dom7b9, as well as -7b5, and diminished 7th chords. At this point all you are really missing is min/maj7, 11ths, and 13ths. Here is the beauty part, if you are playing a C7 or C9, sub the fourth in for the third and now you have a C sus11, or just add an A note on top and now you have the 13th, change that to an Ab and you have a C7b13. This stuff really isn't THAT hard, but if you try to make yourself do more work than you need to it will sure seem like it is!
I apologize for the lenghty post, but this is something that I see students struggle with often and I hope that this helps make you practice time a little more efficient, or easier at least!
