If you wanna get the basics first, you can figure out the timing best like this.
Assuming you use X to jump and R to shield, the buttons are
X (diagonal on the control stick) R
The diagonal can be put in anytime before you push R. You could even just hold down-right/down-left the control stick and just push XR is you want.
To get wavedashes as fast as possible, try this:
Hit X R quickly. You should jump, then airdodge. You wanna get the airdodge as close to the ground as you can to get the best wavedash. Then hold down and do it. You will wave dash in place.
When you learn to wavedash with other characters, this is the timing that is different. All of the characters have different start up frames when they jump. Samus' is pretty short, but not as short as Foxes. For Fox, you should have to go from X to R quicker, but for a character with a long jump time, you would go slow, like Ganon's jump.
Now to add the airdodge, just jump high into the air. At the peak of your jump, air dodge diagonally down. Just move that lower until you get to the floor and you will be wavedashing.
If you just jump, then you pushed R too soon. If you look like you wavedashed, but stay in the same spot, then you didn't push the control stick in time. If you air dodge, but are too high in the air to slide, just speed up your fingers. If you roll, then you did everything backwards.
And just do you know, If you want the furthest distance for your WD, the control stick shouldn't be perfectly diagonal, it should be a bit higher. If you wanna figure out the pefect spot, start at horizontal and hit X R. Slightly lower it until you actually do a wavedash. If you X R timing is right, then you have it as far as you can get it.
On Final Destination, if you just hold down-right, you will cross the stage with about 9 wavedashes. If you get them as far as they can go, it should take about 5.
