The short of it is: Make them a part of the conversation.
As open-minded and inclusive as many individuals in the Smash community are, it's best to think of the overall community as highly exclusive, where serious discussion is for members only.
If you want your character to become a member, you need to first find a way to get in. It's very important to recognize what this is about. Exclusion from the most insulated parts of the conversation is in no way a mark that a character is bad or even unpopular. Instead, it only means that the collective does not view them as valid desires. Because of this, people will hesitate to publicly support or show support for something that may malign them with their peers. Therefore, it's not only possible, but probable that there are many people who would, as the more jaded folk would say, "jump on the bandwagon" as soon as it became communally acceptable to do so.
So how do you accomplish this?
If, like Skull Kid, you get an extreme shot of speculation fuel in the form the ensemble background in the August Direct, as well as a seeming replacement for your assist trophy, these will serve to convince a large subset of the community that you have a credible chance for inclusion, which will automatically get you in the door through brute force.
However, if you don't have such things going for you? You have work to do. I'll repeat that, because it's important: You have work to do. It will be an uphill battle, and one you will lose if you aren't active. You will face adversity by many people who will simply dismiss you as a rabid fan spouting utter nonsense. You will not convince these people, as they will begin the exchange with no intent to converse or listen. So don't try, they aren't your audience, at least not yet.
You're going to need to gather evidence, create things people can look at, and engage in banter, all to get the idea of your character in their mind. In addition, it's important to be smart about how you promote. Do it too often, and you're back at "rabid fan." Do it too little, and no one will remember you.
You're also going to need to do more than just promote, you're going to have to become part of the conversation. Talk about things that aren't related to your character. Join the community, get your face out there, and build a reputation. Think of it as a sort of currency. If you don't have enough rep-points in a community, you won't be able to spend the necessary amount to introduce something that disrupts the status quo with any hopes of being taken seriously. You can't buy reputation on credit, you have to earn it. As a bit of an optimistic note, if you do this right, you can gain reputation by spending it, so if you get a decent base, you should be alright to push from there. Just try to keep in mind that you can go overboard spending too much at once, else you may find yourself suddenly bankrupt, else in severe debt (IE actively maligned).
Find some people willing to engage in that conversation. Remember that, conversation. As a lone voice can do little. You aren't looking for a stage on which to speak your piece to a passive audience, you're looking to get people talking. Talking when you're around and, eventually, talking when you aren't. In this way, a name becomes more visible, and thus more accepted. Once you're accepted in that fashion, those people on the fence about whether to support, reject, or remain neutral will be put in a position to choose. That's what you're trying to accomplish. That's your end game. You aren't trying to get people to affirm your desire in this case, you're just trying to stuff your foot in the door long enough to earn the "Maybe" you need to be let in.
After that, you'll no longer have any real control over the way that character is pushed for or against. It'll be it's own beast like the others, and it's unlikely anyone will thank or remember either you or your efforts.
That's how.
(Don't mind me, just giving an impassioned, detailed answer to what was most likely a frivolous question.)