The line buttons are the ones that are positioned on the right of the screen.
Tap the softkey more on the phone till you can see the softkey apps along the bottom.
Highlight "Pref." with the arrows on the phone. Press the softkey Select.
Navigate to "Buttons", press the softkey modify, Then press the softkey modify with line buttons highlighted.
Select the button required with the arrows and press the softkey modify again.
Select the function you wish to program by highlighting it with the arrow keys, then press the softkey select then fill out the details as required.
Press softkey done when you have entered the required details.
To cycle between your newly programmed line buttons, press the up or down arrow on the phone handset, the page you are on is indicated by the 3 small dots between the buttons on the screen. (They can be seen on the first image above)
Note: When entering letters you will need to press the button multiple times to get the other letters displayed on the button. Capitals and symbols can be selected via the second softkey.
There are many different functions that these programmable buttons can be used for. The following link outlines what can be configured on these phones. What functions can be programmed onto the programmable buttons on Huawei phones?