imho everybody's ocs are mary sues until they really start developing them. case and point: me
I always remembered my posts on this thread being cringe because I thought that what I wrote there was really outdated (character-wise). I proved myself wrong. nice job me.
But as far as oc building goes--here's my process. the steps don't have to be followed in order tbh. I'll use sigmund as an example since she's new.
step 1: get a foundation. This is usually like an inspiration to make an oc. Personally, it just jumps from my mind. I don't really get it from outside sources but it might be different for you. I think up like this core identity of them that you are building off of. For Sigmund, I knew that this oc would be 1) female 2) used an axe as a weapon 3) strong.
step 2: From there, I thought about where she was in the world and in the timeline of the story. Without getting sidetracked, the timeline consists of 3 arcs and revolves around Jr. Sigmund would be a permanent main character in Arc 3. I knew she would have some type of close relationship with Jr and that she would be a reoccurring character after arc 3 finishes and Jr leaves Lyricia (the continent) to the island he was born in.
step 3: appearance. Now I go dig around and design her. I ask myself, where does she live (see step 2)? What's her environment? Do I have a vibe I'm after? I liked Astrid (HTTYD) and Brigitte's (OW) designs. But, since she has an aboriginal ethnicity and she lives in a biome with Arizona + semi-lush grassland oasis vibes, she would physically appear Native American. Remember my base note of strength? I'd rather it be natural strength, so she physically won't be ripped. muscle definition? definitely. 6 pack? No. I liked HZD aesthetic armor and clothing and kept that in mind when thinking about outfits. I kept a natural color palette with a pop of more saturated color so her palette would be mary-sueish.
step 4 [present]: personality.
hmmmm well, the fun isn't over yet. But remember for personality: every positive has a corresponding negative. Often times the positive is the corresponding negative. Sue personalities don't consider consequences. They have personalities with polar opposite traits and no realistic reason why. I mean, shy people can be outgoing--but it's a different situation. I have a rough idea that Sigmund is not a polar opposite, but she is not too similar to Jr. She's extroverted, she looks before she leaps, she's attracted to a challenge and she's definitely a leader type. She has an actual interest in politics and isn't shy about talking about her opinion. She's a community person, she enjoys being around the company of people--and she doesn't mind busy city life. She is not a tree hugger. From these facts I can pull out traits: brave, stubborn, outspoken, family-orientated, more or less on the selfless side, calculated, organized, not overly empathetic, etc. It helps to compare and cotrast against other preexisting characters to help establish things. I don't really think about personality traits as completely positive or negative--anything can be anything depending on the situation. It's just a human trait.
step 5: refine and add. Sense of humor, likes and dislikes, etc. Just play around and see what sticks to them.