Please follow the step-by-step guide to sing-in to iCloud with an existing Apple-id on your iPad :

Step 1: Launch the "Settings" app.

Step 2: Click on "sign in to your iPad" at the top of the screen.

Step 3: Enter the existing email id and password associated with your Apple ID.

Step 4: Click on Sign In.

Launch settings, then tap Sign in, then enter Apple ID and password