In order to safely learn to sing lower notes, you will first need to find the lowest note you can currently sing. Either use a pre-recorded tool online or have a partner play notes on a piano. Starting from C4, try to sing that note. Work your way down until you reach a note you either cannot match at all or that you strain to sing. The previous note is the bottom of your current range. Because it can be difficult to judge our own voices accurately, it is especially helpful to have a vocal coach or someone else with musical training for this step. Or, use an app like SingScope if you need help. Try your best to work on just the next lowest note after the bottom of your vocal range. Practice with a scale that includes just a few other notes along with the one you're working on. Perform these scales daily for roughly half an hour. Stop the exercise if your voice begins to strain.  Practice relaxing into low notes each day to increase your vocal flexibility. The vocal cords are muscles, so frequent practice can help strengthen them, allowing you to sing lower notes over time.  For example, if C2 is the lowest note you have currently mastered, try singing B1 next. Before going to the next lower note, it's important that you can consistently hit your new note. If you can't comfortably sing one note lower, you will not be able to perform the note below it. If you're finding that your voice breaks frequently during these exercises, you will likely benefit from revisiting a higher note and working on that one first.

TL;DR in Indonesian:
Tentukan dasar jangkauan suara Anda. Mulai perlahan. Sempurnakan nada sebelum melanjutkan.