To tune the D string on a Cuatro, you will need a reference pitch to compare it to. This can be a pitch from a tuning fork, an electronic tuner, or the pitch of the A string on your Cuatro (if it is already in tune).
Once you have a reference pitch, place your left hand on the neck of the Cuatro and hold down the A string at the fifth fret. This will create a pitch that is the same as the open D string pitch.
Next, pluck the D string with your right hand, and listen carefully to the pitch. If the pitch is higher than the reference pitch, you will need to loosen the string by turning the tuning key for the D string in a counterclockwise direction. This will cause the string to become less tight, which will lower its pitch. If the pitch is lower than the reference pitch, you will need to tighten the string by turning the tuning key in a clockwise direction. This will make the string tighter, which will raise its pitch.
Keep adjusting the pitch of the D string and comparing it to your reference pitch until they match. It may take a few adjustments to get the string in tune, so be patient and take your time. Once the D string is in tune, you can move on to the other strings on your Cuatro and repeat the process to get them in tune.