Fox Guide

At the start of the game always do a barrel roll!
But seriously, the best thing to do at the beginning of the match is to run up to your opponent and land several up tilts, if done correctly after about 4 hits your opponent will go into the air and be vulnerable to a deadly up air. It's possible to rack up at least 50% early with this tactic.
When playing fox you want to land lots of dash attacks because then your opponent will be in the air and vulnerable to your very strong aerial moves, the same thing goes for grabs.
The best way to prevent your opponent from getting back on stage is by positioning yourself at the edge and as they come up to the ledge perform a down smash, I have managed to KO opponents at very low percentages with this maneuver
When your opponent gets to a higher percentage i.e. above 100% try to KO with an up smash, in order to do this start playing defensively and force them to miss an attack with a lot of recovery lag, after they miss roll around them and quickly KO them with your up smash
If you ever find yourself losing a match the best way to come back is by having strong stage presence (remaining at the middle of the stage) and grabbing doing both of these things will help you dictate the fight leading to easy combos

I hope that this guide was helpful