I'm working on my own mod called Magic Melee (shameless self-plug) using crazyhand, so it really depends on what you are specifically trying to accomplish. The main things to know are:
1. Hitboxes
2. Timers
3. IASA
So Hitboxes are pretty cool, let me explain what all the numbers are in them:
Base Knockback: All enemies will be knocked back with this much force, no matter what % they are at.
Knockback Growth: The amount that knockback increases, seems to work by multiplying base knockback, so for example, low base will lead to low growth, even if you have a large number here.
WDSK: Weight-dependent set knockback, different enemies will be knocked back vastly different distances based on their weight attribute. Do not use this in combination with Base Knockback (keep one of these two at 0) or else it will perform strangely.
Damage: How much % the move deals. Moves can stale if you use them repeatedly, keep this in mind while testing. EDIT: Since writing this, I have learned that increasing damage also increases knockback distance. I'm not sure of it's exact relation, but it appears to be the same as adding base knockback. This is why moves like jabs that have 0 base knockback / WDSK still work.
Shield Damage: Straight forward, controls how much getting hit by this move will shrink a shield.
Size: A larger number here will make the hitbox bigger. Use Develop mode to see hitboxes, by choosing Tournament Mode and switching from Master to Develop. Then hold down the right trigger and hit up on the D pad to see the hitboxes. Press start to freezeframe and tap Z to advance by one frame, invaluable for counting frames for timers, which I'll get to soon.
Angle: The direction you get knocked away in. 0 is horizontally away, 90 is straight up, 180 is backwards, and 270 is a meteor. BUT 361 is a special angle called the Sakurai angle, which means the angle changes based on how much knockback is experienced.
XYZ Offsets: Controls where the hitbox will appear. X and Y seem to always be aligned with the screen (not 100% sure), X is left / right and Y is up / down. Z functions differently, it follows the length of the bone it is attached to. This means you can easily make a sword, for example, "longer" by increasing the hitbox's Z offset, and it will work no matter which way the sword is swung.
Attribute: Some of these are cosmetic (fire, slash, normal), others have a function (ice, lightning, cape). Don't pick Unknown unless it was already Unknown, in which case don't touch it.
Bone: Each body part / joint has a number assigned to it, this effects the "starting point" for the hitbox.
Sound FX: I haven't found a list for all of the possible sound effects usable here, but the list is different from the standalone Sound Effect [12 bytes] thing, if that makes sense.
The last three, I don't know what they do yet... It wouldn't be worth speculating here, lol, but I think the Hitbox ID allows for replacing a hitbox without needing to Terminate Collisions and start a new one.
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Timers... How valuable! They tell your code when to start and stop doing stuff!
Asynchronous Timers: These start counting from the very beginning of the move to the very end without interruption. If you want a hitbox to start on frame 10, just get one of these and type in 10!!
Synchronous Timers: These start counting as soon as their spot in the code is reached, so if you want a hitbox to last for 5 frames after it starts, put one of these and type 5, followed by Terminate Collisions, usually, which turns the hitboxes off.
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IASA, interruptible as soon as. This means you can start walking away as soon as the timer on this function is up! Well, not specifically walking, depending on the IASA function, you might only be able to jump out, for example. Some moves might require FSM mods to speed up the animation, though, since canceling the recovery can look weird.
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I wrote this early in the morning so my answers might be a bit strange, but hopefully this will give you all the tools you need to start modding with finesse! You'll probably run into problems that need more specific answers, but this should be a good start! Check my previous posts, as I may have already asked some of the questions you will be asking.