Superpowers are described using a new element of your character sheet. These are light-weight Extras, an idea introduced in Fate Core, and are called power facts.
Power facts are an enumerated list of narrative permissions conveyed by one or more of these aspects. Like any other Fate Accelerated game, your aspects establish facts. You define your powers by writing down these facts ahead of time.
Your list of power facts should have 3 to 10 items. Each fact can be as detailed or as vague as you want. Some groups will prefer very specific definitions for a given power ("I can run at 320 mph"), while others will be comfortable with generalities ("I can run at super-speeds"). If the group discovers that a power fact - particularly things like immunity or invulnerability - is too vague or too specific, the power fact can be edited during play with the GM's permission.
Characters can have a variety of specific abilities - wizards and their spells, or gadgeteers and their inventions, for example. It's not necessary to list every single specific ability, unless they are a signature part of the character or are likely to see frequent use. For example, a gadgeteer who wears a battlesuit can list both "Armored battlesuit" and "Able to invent small useful devices" as power facts. Similarly, a wizard might have "Air Arrow" (his signature spell), "Conjured elemental servants" (a commonly used power), and "Wielder of elemental magic" as power facts.