Put the button battery in the rear compartment of the housing and off you go! If the robot vibrates, everything has been done correctly. Please insert as far as possible to avoid a short circuit or loose contact. Then the cables can be Hot glue are fixed so that they no longer slide out.
If your battery has run out (and also your spare battery), please dispose of it properly. You can get cheap replacements in various stores such as 1 euro shops. Which battery you need is written directly on the battery.