If you have finished reskin the application and want to publish your application on Play Store, to sign your app in release mode in Android Studio, follow these steps :
On the menu bar, click Build → Generate Signed Bundle / APK → APK
On the Generate Signed Bundle or APK Wizard window, click Create new... to create your new keystore or If you already have a keystore, select Choose existing...
In the New Key Store window, provide the required information, on the Certificate form you must at least fill in First and Last Name, although other fields are optional, it is better if you fill in all fields, your key must be valid for at least 25 years, so you can Sign application updates with the same key for the life of your application.
On the Generate Signed Bundle or APK Wizard window, select a keystore, a private key, and enter the passwords for both. Then click Next
On the next window, select a destination for the signed Bundle.
Click Finish and the signed bundle will generated
