If the answer is yes then you may proceed.
As most of us are likely aware of by now, the Undertale fandom is practically overflowing with skeleton OCs, hence why this is a guide dedicated to just skeletons. Unfortunately, majority of these skeletons are literally just Sans ripoffs (and on occasion, Papyrus ripoffs) with no real purpose other than poor quality romantic plot devices for people's personal indulgence. You don't want to be Sans ripoff #9001, do you? Then here is the biggest and possibly the most important tip I can offer you.
DON'T BE A LAZYBONES. YES, IT'S THAT SIMPLE FOLKS!
Now that that's out of the way, shall we move on to clichés to avoid? Great.
Ah yes, clichés. Often the universal bane of most fan characters. Undertale skeletons fall victim to these easy under the wrong person's imagination.
⤍ SKELETONS ARE ALWAYS HUMANOID. WHY?
Did you know, humans are not the only organism on Earth with a skeletal system? Birds, fish, reptiles, mammals - there are a LOT of kinds of skeletons to choose from! Why should you limit yourself to a human skeleton, when you could be a goddamned TYRANNOSAURUS? Heck, you could be a skeleton dragon! The possibilities are almost endless as nearly every living creature has a skeleton. If you want to be extra unique™ you could even be a mix of bones or a skeleton of an unknown creature. But don't go overboard with that, yeah buddy?
⤍ SKELETONS ALWAYS HAVE BONE ATTACKS. HOW FUN.
Yeah, they're skeletons. Of course they're related to bones. But that doesn't mean you have to have bone attacks. Does your skeleton like pickaxes (not a Minecraft reference I swear)? Maybe they like tridents? Or even simply spoons. These are all valid weaponry. Your skeleton throws rocks at people? Sure, go for it! They prefer to chill their enemies to the bone with ice magic? Also pretty damn valid friendo! You can have bone attacks, but that doesn't mean you shouldn't get a little creative with it.
⤍ SKELETONS HAVE AT LEAST ONE GLOWY EYE™
Wow you have such beautiful eyesock- hold up. Is that. A glowing. Eye. WHAT? ARE YOU SERIOUSLY THAT LAZY? You are literally automatically a Sans ripoff if you do this. There is no excuse. Unless you have a VERY VERY GOOD REASON for it. But even then I'd suggest you seriously don't give your skeleton a glowy eye. Just don't do it please. Give them glowing eye contacts or something instead if they want to cosplay Sans. Maybe even give them two glowing eyes instead, which don't rapidly flash and may be linked to weak magic.
⤍ SKELETONS CAN MAKE MAGIC TONGUES AND HAIR (AND GENITALS, APPARENTLY).
Newsflash - not every skeleton has Sans' kind of magic. Papyrus never demonstrated this kind of magic. Not to mention, since when has Sans EVER made himself a tongue or a glowing blue lightsaber stick out of his pants? It's fanon, not canon, and definitely not a rule or something exclusive to skeletons. If your skeleton wants hair wigs kind of exist, you know? I think a skeleton wearing a wig would be pretty cute.
⤍ SKELETONS ARE ALWAYS NAMED AFTER FONTS
Nope. I'm fairly certain Papyrus and Sans were... I dunno, unique like that. Did you ever see any other characters in Undertale with unique font besides the two skelebros? I didn't think so. Though I don't find this to be a strict "no don't do that" so if you really want and if you have a good reason for it (don't make them another brother/sister/family member please), you can name them after a font. Though I'd still kind of advise against it since it's pretty overused.
⤍ SKELETONS HAVE THIER OWN LITTLE GASTER BLASTERS HOW CUTE
Also a big no-no, since it's exclusive to Sans. And half of these fan-made Gaster Blasters are always just cruddy recolours with random horns or something stuck on 'em. We don't even know the true origin of a Gaster Blaster or how Sans came to obtain them. So overall, don't do that.
⤍ SKELETONS CAN BE HALF HUMAN
Don't. There is not much of an in between besides a rotting corpse. That means your character shouldn't be half-human half-skeleton, but rather a zombie.
⤍ MOST SKELETONS ARE SMALL, CHUBBY, HAVE PUPILS AND HAVE A CONSTANT GRIN
If you do two-three or more of these then you're probably a Sans ripoff. Pretty simple. There are tons of body shapes you can work with and again, OTHER CREATURES HAVE SKELETONS TOO.
Aaand, I think that's it! So to summarise it in basic points if you were too lazy to read through all my hard work...
⤍ A skeleton is not limited to looking humanoid. You can be a freakin' dinosaur if you want!
⤍ Not every skeleton's attacks have to involve bones. If your skeleton likes assaulting people with spoons go for it.
⤍ Sans magic is pretty much exclusive thing to him. There's nothing that appears to hint it's a species-wide thing. Avoid using one glowy eye, giving your character the ability to summon a magic lightsaber from their pants or give themselves glowing hair, and having their own gaster blaster.
⤍ Skeletons don't have to be named after fonts. It's a pretty big cliché, but not a strict "don't do that".
⤍ Is your skeleton small, chubby, and has constant grin? If yes, then they're Sans. You lazybones. Don't be Sans #666.