Before I start explaining, some of these things I'll probably get wrong. If/When I do, PLEASE use constructive criticism. This will not be a perfect guide but may give out a few tips for those who are interested, or are already using Little Mac.
(Play style) Little Mac is a bit of a bait, punish, and pressure character from very fast moves with small amounts of end lag, but he's more so a defensive character, you can't just rush down the opponent and expect to win, so patience is vital. You'll want stage control so that you can be towards the middle of the stage so that you have a better chance of recovering.
(What to do) As mentioned above, Little Mac is a very defensive character and requires patience as well as being able to read the opponent well and knowing what moves to use and when depending on the opponent's play style(s) in order to pressure, bait, and punish.
(Jab, Tilts, and Dash attack) As mentioned above, Little Mac's moves are very fast having less than 10 frames on their first active hurt-box frame and have small amounts of cool-down, so these can be abused quite a bit.
Jab come out on frame 1 (tied with ZSS), and has 2 forms: the rapid jab, and the gentleman (3 hits), use the rapid jab when in a bad situation near the opponent so that if they run into it you have control of the stage, I don't know the use of the gentleman though nor have I actually used it too much. F-Tilt (Frame 4 and 12) is a 2-hit move that has good knockback for a quick spacing tool and can kill at higher percents near ledge. Up Tilt (Frame 5) can combo into itself at very low percents and can combo into a few aerials for that extra percentage, but D-Tilt (Frame 3) is much better with starting combos, fast fallers beware, at low percents the move can combo into itself because of its low knockback and into the other tilts. A combo that works for me quite well is D-Tilt, F-Tilt, running Up Smash (Sweetspot), and then Up-B, another is D-Tilt, F-Smash up tilted, and then Up-B. Dash attack, despite how many people say don't open with it, it's a good move but only in some situations: Not when the opponent is on the other side of the stage, near the ledge, or shielding. Use it when the opponent is near you or is in landing lag if you can't hit them with anything else.
(Smash Attacks) They hit hard, come out quickly, kill early, and beat other moves due to super armor. These attacks are nothing to mess with.
F-Smash comes out the latest of the smash attacks at frame 14 with super armor at frame 8-15 (15 for the downward version with armor at frame 9-15). Yes that is the slowest of them. Little Mac takes a step back before the move hits, altering his hitbox to go backwards. The forward version is a powerful punch sending them more diagonally, the upward version sends them straight upward. Lastly, the downward version does major damage considering how quickly it comes out, and can break shields if the opponent is pressured correctly, but the 'drawback' is that they barely go anywhere in terms of knockback but the 'drawback' makes the move potentially even better since it can follow into a jab-lock caused by Mac's D-air. Up Smash comes out on frame 10, has armor on frames 8-11, and has a powerful sweetspot on the first frame (indicated by fire on the glove). This is your primary kill move from anywhere on the stage and is still fine if you sourspot it. D-Smash comes out (in the front) on frame 10, has super armor on frames 7-10 and it also comes out (in the back) on frame 17, and has super armor on frames 15-17.
This move sends the opponent nearly nowhere vertically but instead horizontally, time it right and you will hit an opponent that's about to grab the ledge.
(Grab throws and Aerials) Not the best but not useless. These are your easy to use mix-up options that will catch the opponent off guard and possibly make a game changer. The throws are nothing special, so if you ever grab them, pummel 2 maybe 3 times and then throw. The aerials are much better though IMO since these can cancel combos, gimp, and jab lock. D-air was kinda explained with the smash attacks for the jab lock since it's a very weak meteor, so that's really about it. F-air and B-air send the opponent similarly to D-Smash, which is why they can gimp characters with bad recovery moves (Mac, Ganon, Falcon, Mario, Bayonetta once she's out, ect). Lastly there's N-air, which can potentially cancel combos or just a move in general. N-air can be combined with footstools to make an early kill combo which ends with Up-B. It's also the fastest aerial in the game at frame 2. No other move is faster, or equal to this move in terms of frame data.
(Specials) Neutral-B is a chargeable attack that kills very early if fully charged and has what I like to call: Hyper Armor. 'Hyper Armor' gives Little Mac something similar to Super Armor, where he won't flinch if attacked... as long as the damage does less than 8% of damage. For example: if Sheik uses F-air, Little Mac will still charge the move, but if she uses Down-B instead, Mac will stop charging the move and be launched. Side-B is for chasing the enemy when they roll (input roll, get up roll, tech roll) just don't use it to recover against Mario, or else Little Mac won't need to take a shower after the match. Up-B is another recovery move and an aerial kill option. What I like to do is hit them once with Up-B, then bait an air dodge closer to the ground and punish with a smash attack. Down-B has active counter frames from 4-26 and if Mac counters a move, he's invincible until the attack reaches it's first hitbox active frame. KO Punch... Don't whiff it please. This move kills extremely early, much like the reversed Warlock Punch, but this has a true combo that can 0-death: D-tilt to a percent about 12-28% depending on weight class then KO Punch. It also has a very infamous wind-box.
(Recovery tips and tricks) First of all, second best wall jump in the game. No really, it is. Look it up. Both Up-B and Side-B don't snap onto ledge until the move releases the final hit, so be wary of this against Mario. KO-punch: I know what some of you are thinking: Why the hell would you use up your most powerful attack just to recover and how would that even work?! Well, the KO-punch when used sends you a bit forward when used on the ground, but in the air you go MUCH farther. Don't use it immediately since you will need some forward momentum after hitstun, this move also has a lot of end lag so you have to use it accordingly to stage terrain since you also have that amazing wall jump. Much higher up if there's no useful wall jump space, and a bit lower to surprise the opponent during their early celebration if their's a wall jump for recovery (something like Battlefield doesn't have a good wall jump for recovery even though one is available, but in something like Omega Wily Castle, or the first Castle Siege area). Here's a link to an example of how much it really helps.
https://www.youtube.com/watch?v=wh2yV74F2PI
A better quality video to explain the KO punch recovery.
https://www.youtube.com/watch?v=uYDc-aZG85k
So to recap: Little Mac is a defensive character, D-tilt is the main combo starter, D-tilted F-smash can break shields, aerials and grabs aren't useless and can surprise opponents, Neutral-B can help even when uncharged, recovery moves don't snap on ledge until attack is finished, and KO punch will help in recovering.