With dash dancing, there are a few main things to always keep in mind, namely the range of Fox's dash>Shffl Nair (which is quite long, thanks to his amazing dash speed) and the range of your opponent's fastest/longest approach.
You generally want to dash dance so that you are in the Nair range but just barely outside of your opponent's longest fast approach (Or in a range where you can consistently avoid their attack on reaction and then punish.
A good way to think of it is that they are always doing that attack, so you need to always move back and forth to avoid it. but still be close enough to get that dash>nair in a moment's notice.
When dash dancing, something I like to do is vary the length of my dash dance so it looks like i am going in, but then I wavedash back out of their range. be smart, don't play at a bad range. don't get grabbed, and don't attack shields without knowing how to stop shield grabs.
When you think it is time to go in, Fox is blessed with an approach that is extremely safe. This is the SHFFL Nair > Shine.
It takes practice to get the timing right and consistent, and when you first try you might have a lot of accidental sideB suicides, but it will be worth it to learn this. It is fox's BnB approach.
When you start off, and also as a good mixup, you can do SHFFL Nair > Jab (you can grab, jab again, do a down.tilt etc.
The reason you do the shine or the jabs after your aerial is to prevent shield grabs. These moves come out so fast that the shielding character will get hit if they try to grab, so you still get a good hit if they try to counterattack, and you don't get grabbed.. The jab after an aerial is a very good option, I use it to set up for a grab sometimes and it can really trip up someone's defense,and the second jab is a good way to mix it up.
Dair>shine is also a pretty good approach, Bair is good too.
If they flinch, you can sometimes run up and raw upsmash them, this one is situational but it really gets in people's heads. They usually flinch most when they think an upsmash is coming; Fox is terrifying when you are at like 100% and he is dash dancing outside of your shield.
But if they stay in that shield, just grab them and up throw up air (depends on character). That also gets in their head, since now they just died because they shielded.
If they attack, which is quite possible, you can shield, roll away, spot dodge, or wavedash back to avoid it. Shield or wavedash are usually what I do, you can attack them sooner with those. You will figure out what to do to punish them as you play more fox.
Also look up on youtube "smash tutorial #1 Spacing" It is an excellent lesson and it uses fox as the example. You should watch the whole series, HMW really did a service to the smash community with those vids.
Keep your opponent on their toes by mixing up your attacks, maybe do a Dair>grab in a game where you had been doing dair>shine, they think you will shine instead so they keep on shielding, but you just straight up grab them (or they get hit, then the Dair>grab often still works great) You can do empty jumps or
If you work on the basic fox techniques and spacing, and really try to apply it in your game, you will have the tools you need to play a pretty decent fox.
Hope you have fun with Fox, he can be tricky to learn but he is very fun and can do some really cool stuff. (look up a Silent Wolf tech skill vid for melee, you'll see what I mean)
TLDR: Go to the melee character forums, read the fox tutorials, practice and use what you learn in your gameplay. They have everything there. Watch tutorial. do mixups. SW does cool stuff with fox. Have fun.
