Disclaimer: Fanfiction is for fun times and it doesn't have to be "good" to exist and be shared!! Also just like all advice in the world YMMV (your mileage may vary) and the fact is that the best writing advice is whatever gets you writing!
STEP ONE of good characterization:
YOU ARE NOT THE CHARACTER. THE CHARACTER IS NOT YOU. STOP RELATING TO THE CHARACTER.
They do not think the way you think. They will not react the way you react. They don't have your morals, preferences, tastes, fashion sense, etc etc etc.
You must appreciate them as a separate person. Only once you have separated them from yourself in your heart can you start on the road to knowing your blorbo....
STEP TWO of good characterization:
When asking any question about the character at all, START FROM CANON.
If the exact answer is not in the canon, pick the CLOSEST scenario / fact / interaction, and work from there.
Example: Blorbo has never seen Glup Shitto held at gunpoint. But Blorbo was really angry when Glup Shitto's diary was stolen, and freaked out during another life or death situation.
Held at gunpoint is like 10x more fucked up than the diary so make Blorbo's like 10x angrier and similarly freaked out.
STEP THREE of good characterization:
For details that are not addressed at all in canon, use THEMES, GENRE, and CHARACTER ARCHETYPE/ROLE to create a believable rendition of the character.
Don't forget to include any quirks that significantly set them apart from similar characters when choosing these things btw.
Your BRILLIANT POWERFUL SCIENTIST may have dark polished hardwood furniture that speaks to sophistication but show her love of travel with tchotchkes from around the world.
Your ANGTSY MANPAIN MAN may choose military combat gear but wear pink socks because his daughter picked them out.
Your MANIC PIXIE DREAM GUY may love jogging with his cute dog in the park but reveals hints of a DARK PAST in his music playlist. And in the gun he keeps in his fanny pack.
Don't forget that other characters likely have their own knowledge and opinions of blorbo that you can draw from. If Glup Shitto is consistently wrong about blorbo and thinks blorbo is probably a good cook, make blorbo a bad cook. If Glup Shitto is blorbo's childhood best friend and used to play doll murder courthouse a lot, it's a good stretch to say that blorbo may have played doll murder courthouse as a child too, and this was something they did together.
STEP FOUR of good characterization:
The character almost certainly hasn't taken the exact life path as you. If they are from a different area, look into cultural norms that may differ from yours.
No one is expecting you to be the foremost scholar on Japanese etiquette or British regional slang, but a little can go a long way.
People in places with socialized healthcare should not be worrying about medical bills, blorbos in tropical areas will not experience the seasons as much as you might, other cultures attitudes towards nudity/sex/drugs may be wildly different to yours.
IF you actually have relevant experience then feel free to use it in your fics. But don't forget that you are not the guy! Really think through how their perspective may differ even in the same situation.
STEP FIVE of good characterization:
PUT LOVE INTO IT AND HAVE FUN!!!
If you don't love Glup Shitto, don't write about them! If you're not having fun, hit the bricks!
Fanfiction is for HAVING A GOOD TIME and SHARING WITH OTHER FREAKS!!!!!!