Worship
Havan
Offerings into fire: a yajna at home or in the courtyard.
When
On a sanskar, a griha pravesh, or the day of a particular sankalp.
A havan needs a kund, samidha, ghee, and the prescribed samagri. Swaha is said with each offering. Fire as witness sits at the centre of Sanatan rite. The wedding fire and a household havan are two forms of one feeling.
Practical care is part of the rite. Burn in a ventilated place, keep smoke off children and the unwell, and leave only after the fire is out. In a flat, a small kund and modest materials are the right measure.
This is a general introduction. Sequence, materials, and timing change with family, region, and shakha. Your priest and your own custom have the last word.