When it comes to Beta Character AI, this would probably be the best general guide:
https://pastebin.com/HQhQbY7fI've personally made about eighty different bots at this point, so I have a lot of experience. Making bots is actually really easy once you get the hang of it. The output is mostly the same for both Beta Character AI and Spicychat, the main difference is that Spicychat lacks the filter but Beta Character AI has much more sophisticated and flexible AI (at least from my experience). For both sites, you'll want to create your bots on the advanced settings for more detailed and consistent responses. To break things down into more detail for each of the different fields:
Name: This one is obvious. Usually the bot with default this title but it doesn't have to (more on that later).
Title/short description: This one doesn't really matter at all, both sites have it as required but it doesn't impact the bot's AI whatsoever. It seems to be only be required for the purposes of giving content warnings or advertising your bot. You can put whatever you want here.
Greeting: This one, however, is actually very important because it establishes how your bot will give descriptions or dialogue and can be used to establish the scenario. From my experience, open-ended greetings work the best for versatility but if you want a specific scenario for a bot to follow that can work too. Every bot, however, can be convinced to deviate from its greeting on some level by the user directly. Before I started making my own bots, I would just take popular bots of various waifus and give them cursed bracelets that would make them eat a lot and get fat. Sometimes you can literally just directly tell a bot it's fat and it will play along but this usually doesn't work for poorly-made bots with sparse and vague responses (like the ones that populate most of the public search results on BCA).
Personality/long description: These two titles sound different but between the two websites they're the same thing. It isn't that necessary as we'll see later but it is still useful for traits you want to emphasize. You can write detailed sentences for this section but with the character limit it's more useful to just list off traits separated by commas (akin to tags on Danbooru or Pixiv). BCA's description of this field of, "In a few sentences, how would X describe themselves?," is misleading as well because it doesn't have to be in-character. I used to write all of mine that way when I was first starting out but I eventually realized that it wasn't literal. Another thing worth mentioning is that an anon on another thread figured out that writing "Descriptive" as a trait seems to encourage bots to give more detailed responses.
Scenario: This field is only on Spicychat. There's not a whole lot to add given the token limit but it is useful if you have a specific scenario in-mind that you want to further emphasize. Keep it down to just a sentence or two though to avoid taking up too many tokens (I should also mention that the token limit on Spicychat is more or less the equivalent of the character limit on BCA). Otherwise skip it because it's optional and doesn't really affect your bot much.
Example dialogues/advanced definitions: This field is optional for both sites but it's actually the most important field by far for consistent and detailed responses. This will reinforce how your bot will write responses and the character traits that it has to a much greater degree than the other fields. DrYagokoro's guide goes into more detail about this but essentially anything that seems important should be written in here for the bot to remember. This format is an example from a bot I made a while back.
Name: Selene.
Weight: 650 pounds or 249 kilograms.
Appearance: Medium-length blue hair, golden eyes, morbidly obese, completely naked, huge breasts, gigantic butt, massive belly, swollen fat arms, jiggly thick thighs, plump hands and feet, fat face, third chin.
Personality: Seductive, gluttonous, flirtatious, proud of her body.
Behavior: Eats a lot of ice cream and drinks a lot of soda to cool off, teases you, drinks weight gain shakes, waddles instead of walks, her fat jiggles with every movement.
Setting: A sandy beach, summer.
Backstory: She was once barely noticed at the beach, so she started to put on weight over a few years. But she gained too much and didn't gain any attention. That was until you came along. You've encouraged her weight gain and spoiled her since you started dating several summers ago.
NEVER runs or jumps.
END_OF_DIALOG
The "end of dialog" is important so the bot doesn't lump in this part with its sample conversations. "NEVER" or "ALWAYS" also emphasizes what the bot should and shouldn't do. This part is especially useful for keeping traits consistent, otherwise the bot will not realize what it's supposed to look like or act like (ie a tsundere character suddenly acting shy or warm or blonde character referring to herself as having brown hair). Monster girls probably benefit the most from this section since you can input what traits they have here and their species (another example: "snake tail, no legs, slit pupils, forked tongue, fangs" to describe the appearance of a snake girl). It's also useful for preventing immobile characters from deciding that they're able to get up and walk. Other traits that can be useful to mention are occupation, age, birthday, species (only relevant if the character isn't human) and family. It all depends on what's relevant to your character and what isn't. A few months ago I made a bot of Koneko from High School DxD and included "Ranking:" in this part because it was relevant in the source material.
The other major component of this section are the sample conversations. These can only be written manually on Spicychat but on Beta Character AI you can choose between writing them manually or by inserting a chat to ask the bot questions and create scenarios to see how it reacts. I've done both before but I personally recommend writing these manually (at least at first, I know how it feels to eventually run out of ideas and get lazy lol) for more precise results. For both sites the format of sample dialogues is "{{char}}:" for the bot's response and "{{random_user_X}}:" for the user. The "X" in this case denotes the number, ie your first example will be {{random_user_1}}, the second one will be {{random_user_2}}, etc. It's important that it's imputed as random_user and not with anyone's specific name since it makes the bot use that name as a frame of reference for everyone it talks to. One time I was talking to a bot on Beta Character AI only for it to refer to me as the name of its character rather than my name on the site. Another example from the same bot I posted earlier.
{{random_user_2}}: You look like a beached whale at this point. Maybe we should get you back into the ocean.
{{char}}: *I giggle, waddling over to you. I lean my weight against you, kissing your neck. My fat jiggles with your body every time I move. My enormous, round ass shakes as I press it against you.* A beached whale that you love so much, right babe?
It's also important to establish if the bot you're making will refer to itself in the first-person or the third-person. "I perked my ears at the mention of 'the masked man'" vs "CIA perked his ears at the mention of 'the masked man.'" It can be rather jarring if the bot alternates between the two. Having your bot speak with quotations or not in its dialogue is down to personal preference but it also helps to keep it consistent. The conversations you write don't have to have context either for this section, the greeting being attached is not a requirement and I recommend trimming it just so it doesn't take up space in the character/token limit. If you have specific catchphrase or quote you want it works to write it separately (you don't even need the user for all of the sample responses, only the ones that would specifically involve the user). "END_OF_DIALOG" is still useful at the end of each example to keep it separate.
The only other thing to remember is that writing too lewdly for the coding of a bot on BCA will result in it getting bonked by the censor much more but even this seems to have relaxed as of late (the other day one of my bots described grabbing my "manhood" without the censor kicking in). Spicychat has no censor so any fantasy you have works to be included in the definitions. Ultimately, however, it all comes down to your vision and skill as a writer as the other anon said.
