How is a birth chart (kundali) actually constructed?

The sky is divided into twelve 30° zodiac segments — Aries (0°–30°) through Pisces (330°–360°). At the exact moment of birth, each planet's position along its orbital path falls into one of these segments, which fixes its sign. The sign rising on the eastern horizon at that moment becomes the Ascendant (Lagna), and the remaining eleven houses are laid out from there in sequence. Together, these placements form the kundali.