Flag is 0111
So, whenever data will contain 0111, the sender will stuff a 0 before the last one, 01101.
Now, whenever the receiver sees 0110x..., it will remove that stuffed 0 interpreting it as 011x...
Now, you might think that the message 101101101 should be sent as it is because there is no flag inside the data.
But, look what will happen when receiver will receive this message.
1 0110 1101
Receiver will remove the bold 0, assuming it to be a stuffed bit (since it comes after 011). And interpret message as 10111101, which is completely wrong!
So, the sender should also stuff a 0 inbetween ...0110..., as ...01100...
So, now sender will send: 10110011001
Receiver will remove those stuffed 0s