There are two main methods of superhero/supervillain quick change that writers typically use: the “convenient location” method and the “improvised” method. Both have their pros and cons, but ultimately it comes down to what makes more sense for your story.
The “convenient location” method is when the superhero/supervillain has a hidden lair or safe room where they can change into their costume without being seen. This is a great option if you want to keep the suspense and mystery around your character’s identity intact. However, it can be difficult to come up with a believable reason why your character would have this kind of setup.
The “improvised” method is when the superhero/supervillain changes into their costume on the fly, usually in a public place. This is a great way to add some action and suspense to your story, but it can be hard to make it seem believable that your character would be able to change so quickly without being seen.
Ultimately, it comes down to what makes more sense for your story. If you can find a way to make either method work, then go for it! If not, don’t worry about it too much – there are plenty of other ways to add suspense and action to your story.