pH sensor measurements unstable

[Grumpy_Mike]

@MisterF

Should I place them between the VCC and GND

Yes.

@MisterF

should I place them over the data line from sensor to the arduino?

Only if you want to destroy the fast changes in a signal.

@MisterF

I remember correctly, the ceramic capacitors should be placed closely to the arduino?

Close to the components, that means sensors and other boards.

Electronics is a bit like using LEGO, when the build gets big you have to do extra things to make it stand up.