Just to further emphasis what the other people are saying,
One of the best ways to improve on Smash outside of practice is watching videos, this includes content from top players (in your case Kirby players), tutorial channels such as Beefy Smash Doods, My Smash Corner etc, and watching Tournaments both Live stream and vods (of course it really doesn't matter when you watch it as long as you do. Live streams do tend to help you immerse yourself into the tournament more, or in my opinion it does). Also Smashboards has guides for most Characters in Melee and Smash 4, read them and you can learn a ton about your characters, there is some really useful stuff in there.
You can learn a ton from doing this, even just the simple stuff like combos and strings. You can see what players go for out of certain options etc and then incorporate that into your own gameplay.
Practising is a given, you have to practice. Whether that is CPU, For Glory, Anthers Ladder, LAN with your Friends. All of this helps, but of course there are pros and cons to each.
CPU's don't act like humans, they are programmed to do certain actions based on certain events. This becomes very easy to understand over time and really only helps you develop your own ability to move and character understanding. It does however allow you to test some combos since lvl9 CPU tend to be able to air dodge or perfect shield probably every time. Other than that its not generally a good idea to practice on CPU.
For Glory gets a lot of backlash, but at the end of the day you are playing against real people. This will help you understand what options real humans go for instead of learning patterns of CPU's, this does however limit your practice to Omega and Final Destination stages whilst also and quite commonly introducing lag which can make things usually very safe into extremely unsafe options.
Anthersladder (Or Smashladder) over comes the FG issue, allowing you to play other users in a more competitive environment using legal stages. It also allows you to actually communicate with your opponent which is really useful since you can ask how they found your play and ask for advice (Or even learn to dish out advice since if you can understand how your opponents are going wrong you can better yourself during game). You get to play a real variety of characters as well as players throughout the world assuming they have good enough connection. Again though this comes with its flaws mostly consisting of Lag since its online play but mostly its just a real pain to set up. Since you have to go in and out of online mode to add players. Get over that and you can really make the most out of Anthers as its a really useful tool.
LANing with your friends and other local Smash players. This means you can practice offline as well as tailor your issues with the other players on a more human to human basis. This is your best choice of practice since you get a bit of everything you need for your practice. Downsides include travel, time, money and player availability. It can cost a lot to travel to friends etc to play, it also takes alot of time but it can really benefit you. Although don't think that is be all and end all, since you will learn that player and may play well against them but you do need a variety of players. This is where tournaments come in.
Tournaments are a great way to play and honestly imo one of the best and most enjoyable ways. You get to meet loads of players who enjoy playing smash just as much as you do yourself as well you get to meet players who are potentially better and some who are extremely knowledgeable about the game. Most players are willing to help those who demonstrate there efforts and really enjoy playing and learning. It tends to cost a lot but its a full day (or more) of pure smash!
Last but not least one of the MOST important parts to improving is remembering that your improving yourself. Watch your own replays(there is a really good tool built into the game itself to save and review game play. Ask how you played during sets and what you can do to improve, and focus on your issues and take in what people say about you. You might not always like what people are saying but at the end of the day if someone is saying your doing something wrong you can take that away and learn from it, come back and be a stronger player than you were before.
I guess one last thing would be, always remember that this is a game and to have fun. I know a lot of players that try to improve so much and get to competitive that they lose all love for the game which in turn stop them practising and improving. If you feel frustrated with the game, turn it off, take a break and come back and try and remember the fun things about this game and remember why you love to play etc. even if you play characters you think are bad, or you don't even know how to play. You'd be surprised how much it helps.
Otherwise good luck, remember to have fun and get practising!
WinterShorts
has already stated CPUs aren't really the best way to practice. They're good for certain things like getting certain combos down or practicing techs in your spare time; however they will ultimately give you bad habits if you rely on them.
