Talk to Harvey standing by the long garden just west of Falador, just west of the crumbling wall which requires level 5 Agility to cross. (1•✓)…
What's the matter?
Entering the house
Go to the front door of the house.
Look under the northern potted plant for a key and enter.
Climb down the stairs into the basement.
If you didn't bring leather gloves, search the nearby crates for a pair. You may have to search several times.
While wearing leather gloves, enter the gate.
Open and search the cupboard, straight ahead on the western wall for a magnet.
Go back up the stairs and grab the cheese off of the table.
Go to the room located on the south end of the house.
Use cheese on the mouse hole on the eastern wall.
Use the magnet on the mouse.
Retrieving the ball
Exit the door and make your way to the fountain, being careful not to be seen by the witch.
Strategically walk behind the shrubs to hide, being sure to stand in the middle space of the shrub. If you are caught by Nora T. Hagg you will need to repeat the process involving the magnet and the mouse. Do not try to surge, it will likely result in being spotted.
Check fountain for a key.
Return east down the shrubs to the shed, being careful not to be caught by the witch, and use the key on the door.
Try to take the ball, the experiment will appear and attack you.