Sennheiser CX Plus Instruction Manual page 18

Using the earbuds
18 | CX Plus True Wireless
2. After 20 seconds, the earbuds are paired with each other.
The LED on the charging case then lights up red and green 3x.
The earbuds are now paired with each other.
If you have closed the lid of the charging case too early, repeat
the above procedure.
After the pairing process with a spare earbud, all settings are
reset. Pair the earbuds with your smartphone again (> 16).
1
L
R
2
Keep open
3 x
20s
Connected