Ok so, mechanics of the double jump and DJC. Simple stuff first.
Just like Mewtwo, Peach, and Yoshi, Ness' double jump allows you to change direction and momentum instantaneously. If you run and jump, then hit back and double jump, his direction will change immediately and his forward momentum will be replaced with backward momentum. (An interesting thing to note is that a double jump straight up has a tiny bit of momentum forward. Without touching the analog stick, you can move across FD with just DJC'd nairs, very slowly).
Also like Mewtwo, Peach, and Yoshi, Ness' double jump acceleration is reversed, so that it starts slow and gets faster as it reaches the top. Initiating an attack at any point during the jump cuts this acceleration short, however the momentum of that acceleration up to that point many still drive Ness upward a bit during that attack.
If you combine these basic things with fast falling, air dodging, wavedashing/landing, and auto-canceled dair, you pretty much have all the basic ingredients for agility and maneuverability.
Here are some exercises that I've been doing lately, to help with DJC control. These are mostly offensive exercises, but being well-versed in these will give you the foundation needed for defensive or maneuvering techniques. (There's a lot of repetition here, however I wrote it all out for reference and completeness purposes. Each number is a different exercise, the numbers aren't representing any kind of sequence)
A. Nair
1. Jump > Double Jump (close to the ground) > A > Fastfall > L-cancel. Simple, DJC in place. Repeat as quickly as possible without autocanceling.
2. Analog forward > Jump > Double Jump (close to the ground) > Analog in neutral position > A > Fastfall > L-cancel. This should move you forward with a nair. Repeat as quickly as possible without autocanceling.
3. Same as above with 'Analog back' instead of forward. This should move you backward with a nair. Repeat as quickly as possible without autocanceling.
4-6. Same as 1-3, but after a short hop or full jump. Practice this with jumping neutral, forward, and backward for your initial jump.
7-9. Same as 1-3, but delaying the attack after the second jump so that your momentum pops you right above a platform.
10-12. Same as 1-3, but repeat as quickly as possible WITH autocanceling (and no L-cancel, of course).
B. Bair
1. Jump > Double Jump (close to the ground) > Analog back > A > Fastfall > L-cancel. DJC in place. Repeat as quickly as possible without autocanceling.
2. Analog forward > Jump > Double Jump (close to the ground) > Analog back > A > Fastfall > L-cancel. This should move you forward with a bair. Repeat as quickly as possible without autocanceling.
3. Same as above with 'Analog back' instead of forward in the first step. This should move you backward with a bair. Repeat as quickly as possible without autocanceling.
4-6. Same as 1-3, but after a short hop or full jump. Practice this with jumping neutral, forward, and backward for your initial jump.
7-9. Same as 1-3, but delaying the attack after the second jump so that your momentum pops you right above a platform.
10-12. Same as 1-3, but repeat as quickly as possible WITH autocanceling (and no L-cancel, of course).
C. F-air
NOTE: F-air has five hitboxes. For exercises 1-3, instead of repeating 'as quickly as possible,' try practicing control over how many of these hitboxes actually come out. You can achieve this by either delaying the time between your jump and double jump, the time between the double jump and the attack, or the time between the attack and fast-fall, or a combination of all three. You should aim to consistently only allow 1, 2, 3, 4, or 5 hitboxes to come out before you hit the ground.
1. Jump > Double Jump (close to the ground) > Analog forward > A > Fastfall > L-cancel. DJC in place. Repeat as quickly as possible without autocanceling.
2. Analog forward > Jump > Double Jump (close to the ground) > Analog forward > A > Fastfall > L-cancel. This should move you forward with a f-air. Repeat as quickly as possible without autocanceling.
3. Same as above with 'Analog back' instead of forward in the first step. This should move you backward with a f-air (fadeaway). Repeat as quickly as possible without autocanceling.
4-6. Same as 1-3, but after a short hop or full jump. Practice this with jumping neutral, forward, and backward for your initial jump.
7-9. Same as 1-3, but delaying the attack after the second jump so that your momentum pops you right above a platform.
10-12. Same as 1-3, but repeat as quickly as possible WITH autocanceling (and no L-cancel, of course).
D. Uair
1. Jump > Double Jump (close to the ground) > Analog up > A > Fastfall > L-cancel. DJC in place. Repeat as quickly as possible without autocanceling.
2. Analog forward > Jump > Double Jump (close to the ground) > Analog up > A > Fastfall > L-cancel. This should move you forward with an uair. Repeat as quickly as possible without autocanceling.
3. Same as above with 'Analog back' instead of forward in the first step. This should move you backward with a uair. Repeat as quickly as possible without autocanceling.
4-6. Same as 1-3, but after a short hop or full jump. Practice this with jumping neutral, forward, and backward for your initial jump.
7-9. Same as 1-3, but delaying the attack after the second jump so that your momentum pops you right above a platform.
10-12. Same as 1-3, but repeat as quickly as possible WITH autocanceling (and no L-cancel, of course).
E. Dair
1. Jump > Double Jump (close to the ground) > Analog down > A > Fastfall > L-cancel. DJC in place. Repeat as quickly as possible without autocanceling.
2. Analog forward > Jump > Double Jump (close to the ground) > Analog down > A > Fastfall > L-cancel. This should move you forward with an dair. Repeat as quickly as possible without autocanceling.
3. Same as above with 'Analog back' instead of forward in the first step. This should move you backward with a dair. Repeat as quickly as possible without autocanceling.
4-6. Same as 1-3, but after a short hop or full jump. Practice this with jumping neutral, forward, and backward for your initial jump.
7-9. Same as 1-3, but delaying the attack after the second jump so that your momentum pops you right above a platform.
10-12. Same as 1-6, but repeat as quickly as possible WITH autocanceling (and no L-cancel, of course).
F. Various Advanced stuff
1. Mess around with using forward or backward double-jump momentum from the above exercises to ledge cancel. Some things you can do:
-- Ledge cancel f-air into another attack, with/without a fastfall (can combo into every aerial besides dair, however fast-fall uair seems most useful).
-- Ledge cancel a dair into a fast-fall bair (if they missed the tech, they'll get hit by it).
-- Ledge cancel anything to fast-fall waveland.
2. A technique I call 'Foot Drop':
-- After a jump, or at any point in the air, double jump; the next frame, analog down and A at the same time. This will cause you to fastfall with a dair at any point in the air, even while moving upward. Since dair has autocancel frames until the hitbox comes out, you have zero lag when you land. When used sparingly, this maneuver can surprise an opponent expecting to punish you for full-jump approaching. Unwitting opponents usually shield, which is a free grab. It's smart to save this for clutch moments.
3. By DJCing a dair after a full jump without fastfalling, you move downward very slowly with the hitbox. This makes the move much more likely to shield stab, even if their head is only sticking out a little bit. Another thing to save for clutch moments.
4. Very low bairs can shield stab, especially a weakened shield.
5. By controlling the height of an attack with DJCing, you can shield stab opponents' feet while they're standing on platforms. This can be done from below, and also after dropping through the same platform.
If I can think of more, I'll post it, but that's all for now!