You thought to make your own AU but you don't know how to do it? In that case, I hope this guide will help you a little 😉

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How to start?

Start by thinking up story, find the main thread and expand it. Remember, however, do not get too excited too much so you come up with an interesting idea that doesn't fit into the present moment in your history, and which you necessarily want to introduce now, because it can disrupt the whole creation process. Slowly, step by step, develop the story, but if you really have an amazing idea that cannot get out of your head, write it somewhere in the margin, maybe you can enter it at a later stage of creation.
Keep your script clean and organized. Undertale has a lot of characters to deal with, so in your story you should choose who's going to be your main character, so you should focus on them mostly.
Warning: if you want your AU to be recognizable, search for some popular AUs, to make sure you haven't created something that could be seen as a copy of someone's story, cause it usually happens, sometimes even if you don't know AU people keeps telling you're stealing ideas from.

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How to create designs?

Since you managed to write a rather long script (not necessarily the whole story), you certainly imagined the appearance of the characters there. On this basis, you can create a design. So how do you do it, if you don't know how to draw clothes and your idea of the character is not clear enough? Look for clothes on the internet, you can even model on your own wardrobe! Remeber, outfits needs to fit into your story time (like, if you're makin AU that happens in medieval times, you need to fit in with their clothes, of course)

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How should I name my AU?

Well, the name should be related to the history of your AU. However, most often all English names are occupied, so it requires a lot of creativity to find unique one, especially if you want your AU to be recognizable, the more should it shouldn't be called as a dozen other AUs.
1. Find the best word that describes your AU.
2. Check if there's an AU already named like that
3. If the word you found is already used as a name of other AU:
- try to find similiar word that means the same but is writted differently example: funny, ridiculous, laughable, ludicrous, farcical, preposterous (all are similiar word of "funny")
- use your word in other language, example: Lighttale can be Hikaritale (hikari is light in japenese)
- name your AU after something that's not even similiar with your AU history (optional)

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How to get AU popular?

With the help of a great original story of course. This makes the AU popular: great story and eye-catching character design. The greatest popularity of AU can get first of all: a comic from it, thanks to which you can show the world even better your AU. The second thing is animation: animations attract people as well as comics, they show even more how you see your own AU when you illustrate it in animation.

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How develop character's personalities?

The best way is to use your characters "in practice", is roleplay
Roleplay is usually a conversation between two people or more (which is not recommended), each of which uses their characters to communicate with the characters of the other person, playing scenes of different situations, for example: in danger, quiet day, even nighttime funs or sin scenes.
* Although static roleplay is not always very helpful in character development, it is best to play scenes in which characters are in danger, this formula is usually much more interesting and helps in character development.
Another way is to give yourself to your own character. example: if you like to make a pizza, make your character like it too. Even if you like: collect mugs, make your character do it too.

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Things to avoid the most

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Alternative Universe ≠ Sans
Sans is not your WHOLE Au, no need to make him as main character, there's a lot of characters you can put into that rule. Just because people likes Sans mostly, doesn't mean you should too

Making AU of an AU is NOT original
If you have your favorite AU, DO NOT make alternative thing out of it, it's someone's idea YOU changed, that's not original at all. 

Alternative Glitchtale's AREN'T ORIGINAL either!
Same as above, alt. Glitchtale's are just copies of original GT, and it's simply not cool, since Glitchtale is everywhere 

Undertale is not only about SKELETONS
Yup, there's a lot of other characters like Dreemurrs, Undyne, Alphys, even Mettaton, don't focus on skeletons only

Think outside the box
Even if Undertale AUs usually focuses on Underground/Surface, doesn't mean you can't create something original here too! Who knows, you AU can take place in some mystical word where big purple mushrooms talks 

YOU CAN'T GIVE FRISK/CHARA GENDER!!!
Nope, you can. Why wouldn't you? Just because people are horribly sticked to "genderless" characters for some reason? Frisk is genderless so you can choose a gender in a game, so, what's the hold up?

Welp, if it helped somehow, then I'm glad ^^ please comment if you think I could write about something else here!~