This is my guide to creating Sonic FCs, and I guess you can use it for any fandom.
I'm only making this because of how lazy the community is with creating FCs.

Step 1: Inspiration

The main thing needed for a FC of any fandom. Without this, your FC cannot be created. There are a load of places you can get your inspiration for your characters design and/or personality

-Yourself - You can model your FC after yourself. This won't make them a Mary-Sue, but what will make them a Mary-Sue is making them a self-insert. Use parts of you that you like (or dislike if you want) and use that to craft your FC's personality or even their design
-Others - Take critism. You'll get it a lot, but it's not bad at all. Listen to what others are saying about previous characters you have made and use what they have said to help you flesh out your characters.
-Other FCs - Have a look around the internet at other Sonic FCs. Take a look at the successful ones and see what they have done to the characters. Find a wide variety with varying body builds and species. Look at every aspect of them, especially their backstory and other key information
-Music - I find this works really well. Most people give their FCs theme songs, but then struggle to relate their FCs to songs, so this is a great way to avoid that. Listen to songs lyrics and hidden meanings and try to link it to a part of you (if you are also using point 1) or if there is a song you really like, use the lyrics to create a personality

Step 2: Powers

Most characters in the Sonic universe have powers. Whether it's a kinesis or super-speed or extreme power, they're all important

-Keep the powers limited. Don't give them 3+ mega powers
-Have one main power which makes the character known and stand out
-DO NOT give your character super-speed or Chaos energy. Sonic is known as the 'Fastest thing alive' for a reason and the chaos power is almost god-like

Step 3: Naming

The name of the character! There are a couple of rules when it comes to naming a character, starting with the Forbidden Names.

Forbidden Names:
-Shade - There is already a CANON character called Shade from 'Sonic Chronicles: The Dark Brotherhood'. She's a female terra-cotta echidna.
-Any name sounding similar to Amy or any other character - This really doesn't need to be explained, does it? Names like Nikki and Amelia are really bad within the Sonic Community as they are way too similar to Amy's name.
-Foreign names if they aren't foreign. It's like having a character named Tenshi (Japanese for Angel) and having them with no Japanese heritage. It just doesn't work.

-Have the name relate to the Character. Tails has his two tails. Big has a stocky build. Tikal's home was modelled after Mayan society, specifically Tikal, the remains of an ancient city temple.

Step 4: Make sure they match

It's one thing to have a personality for your FC to have a personality, power and name, but do they match? This is something a lot of people have problems

-Do the name and power/personality match? - Think of the actual Sonic cast with this. Sonic has super-sonic speed. Knuckles has extremely strong fists. Big has a big heart (and has a larger body type). Blaze is a pyrokinetic. Don't give your character a normal name and something like omnipotence (don't give them this anyway)
-Does the name reference the design? - Not all characters do this. Cream's fur is cream. Silver's fur is silver. Shadow's fur is dark like a shadow. You don't have to use a colour though; if your character has a signature scar, they could be called Scar if they were a guy.

Step 5: Backstory

Everyone has one! Whether it's a sad tale of love or a fun-loving childhood, they're all the same in the matter of importance.

-DON'T kill of their parents - most Mary-Sue's have this happen from a young age.
-DON'T make an official character OOC - If an official character becomes out-of-character when around your character, you've done something wrong.
-DON'T have your character date a ton of official characters - refer to the rule above if you want your character to date an official character
-DON'T make it super sad
-Try not to make romance a thing for your character - try to do something like make an Asexual character where they don't need romance or a really shy bookworm
-Have their role have fit their personality - Don't have a princess who's super violent without any influence for violence

Step 6: Designs

Now, the thing people will see! You can be as creative as you want, but make sure you don't go too overboard.

-DON'T make your character all cold colours/warm colours; have a mixture, usually with the eyes or facial features so the eye is drawn to the face. So if your FC ad cold colours for their fur and outfit, give them an eye colour of a warm colour so people are drawn to them
-Make the outfit fit the characters design, personality and power(s)
-Use complementing colours which are soft on the eyes.
-Look at other characters (I look to anime mostly) and see how their outfits are styled
-Try using quills with your deign, but don't make them mirror a canon characters
-Try not to use stripes or highlights. There are too many like this in the fandom and your character won't be noticed
-You can use and colour for your FC' eye colour, but don't combine black fur with red eyes. I gave my main FC lighter fur and one blue eye.
-it's one thing to use an Official Character as inspiration, but it's another thing to slap different colours and an outfit on an Official Character and call it original.