Rest the Spectrolino on the calibration plate with the white reference inserted into the calibration slot. Press the measurement key for about four seconds. The instrument will confirm successful calibration with one short tone, one high long tone, and finally another short tone.
If your calibration is failing, check that the serial number of the white tile matches the serial number of your Spectrolino. Clean the tile with a clean soft cloth and try the calibration again. If it continues to fail, reset your instrument and try again.