How to use Power Apps in SharePoint Online?

1. Open your SharePoint Online List,
2. In the toolbar, select Integrate > PoweApps
3. Finally select, Create an App to create a new app based on SharePoint List using PowerApps.

You can also Customize a SharePoint forms by selecting Customize Forms

SoultionA:

1.Navigation to the list/library as an admin

2.Integrate >> Power Apps >> Create an app/Customize forms

SoultionB:

1.Click "+New" to create new item

2.On the New Form, click Customize with Power Apps

Here is a good document for your reference:

• Create a canvas app from a SharePoint list

Another method:

Go to Power Apps. Create new canvas app. Import the SPO data connection.And begin building this way.

This method is highly customizable although more "difficult"