Powerful characters are awesome! They can be so much fun to write for, roleplay as, and just have in your OC menagerie. Powerful characters can take many forms. They can be physically strong, mentally unbreakable, or have an abundance of magical abilities, but they can also easily cross the line into being overpowered. Read this page if you're not sure what OP characters are.
How to make a powerful character
You need to follow the steps to making an OC when you make any character, powerful or not! However, when making a powerful character, you should take extra consideration on their personality and backstory.
Why do they have their powers?
Were they born with it, or was it caused by some magical accident? Given to them? A magical relic? Are they some kind of deity or god in their world?
How do they use their power?
Are they protective of their small village, power-hungry and ambitious, unsure how to handle it, or just straight up criminal with it?
What powers do they have?
Don't give them their list of powers until you've figured out those two questions! It helps you not to go overboard and cross the line into OP.
Again, not all powerful characters have to have a bunch of magical powers. Think about Marvel's the Avengers - Only a couple of them(Thor and Scarlet Witch come to mind) actually have magical abilities, but all of them are still very powerful.
Every powerful character generally needs some sort of weakness. Everyone knows superheroes are deathly allergic to one thing, right? Superman and Kryptonite?
This is bad writing(though can be pulled off, if done well, as with everything)
Think of power weaknesses as personality flaws. Having just one really bad character trait can make an interesting character, but generally it gets boring and stale.
To make your powerful character more fleshed out, give them a multitude of weaknesses. Try giving them fears, phobias, and aversions, or a multitude of weaknesses instead of just one.
And bear in mind, very often a character flaw is just the flip side of a character virtue. Or it's a virtue taken too far!
Writing a powerful character
Now that you have your powered-up character, how do you write them in your story?
Keep in mind how your character thinks about their powers and what they tend to do with them. If their powers are always present, have them absently use their powers - for example, a super-strong person chucking rocks across a lake ridiculously far or a telekinetic person levitating their book while reading.
Or, in reverse, if they're anxious about their powers, perhaps make them hesitant to use them, lie about having powers, and try to hide any signs of their powers, leading to other characters getting injured or killed due to their actions.
If their powers are unstable, give them accidents or slip ups.
Always think about how they got their powers and what they do with them. Never just pull out their powers for a deus ex machina moment, or to suddenly save the day.
Roleplaying a powerful character
Mostly follow the same advice as the above section! Roleplaying and writing are fairly similar and have a lot of overlap. Again, never just use their powers to save the day. When roleplaying, you can have a lot of fun as your characters interact with and bounce off of other characters.
Perhaps they can feel out of place, being the strongest, or feel jealous, being the weakest in a group.
Stick in character and remember what abilities your character has and how to use them in various situations. Be creative with how you can use your powers - for example, a character with control of light can't physically fight or strike enemies, but they can blind them in a battle, and a character with super speed or elasticity can do household chores much easier!
also, shapeshifters can fit into any clothes, alter their eye colors if they don't like it, and other vain things. they don't always have to be turning into dragons. Think practically, and think of your character's personality!