(The excerpts are taken from the book 'THE GREAT HINDU TRADITION' authored by Sarma Sastrigal.
A more detailed description about 'darbhai' can be found in this book.)

* How to wear the ‘Pavitram’
The darbhai Pavitram is to be worn on the ring finger of the right hand, in a circular formation, prior to starting any religious rite like homa, dana, yaga-yagna – in fact any deva or pitru karma. This ubiquitous adornment on the finger of the karta of any ritual is called a ‘pavitram’, and it is necessarily made of darbhai.

* Can Pavitram be worn permanently:
There is an opinion that one can permanently have the ‘pavitram’ on. Perhaps an extension of this thinking is the idea ascribed to sages like Katyayana and Harita that a pavitram could be made of gold and worn all the time.
Please note that a pavitram made of gold cannot substitute a darbhai. When a karma starts one has to necessarily have a darbhai pavitram on, even if one is already wearing a swarna pavitram. Any reference to pavitram means only darbhai.

* What is meant of tarjani and who can wear it?
Incidentally, when a swarna pavitram is worn on the ring finger, a ring called tarjani made of silver is to be simultaneously worn on the index finger. And only the eldest son can wear the tarjani (but not when his father is alive).

* Who has to prepare Pavitram?
Generally one does not prepare darbhai for oneself. It is either received from the family Purohit (Sastrigal) or a properly qualified elderly person. Of course if the pavitram is not available from these sources one can prepare it oneself. The darbhai has to be fresh and prepared just before the start of a ritual – it should not be stocked.

There are restrictions - Pavitram differes!

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- Pavitram for all occasions are not same. There are restrictions in the number of darbha leaves used to prepare the pavitram.

* In Vedic recitations, meditation and for Deva puja it is prepared from 2 leaves of the grass.
* In all ancestral worship like Sraaddha or Amavasya libations we use 3 leaves.
* In death rites darbhai is prepared with one leaf of the grass.

Though the number of leaves may change, the shape, size and method of preparing it do not change.

* Varieties in the type darbhai:

As we have already seen, there are quite a few options for the leaf to be used as darbhai. The type known as viswamitra is available in plenty. Other grass leaves like that of paddy, wheat, yava, rundhura, and usiram (vlamicheri ver) can also be used.

* Regulations for wearing & discarding the pavitram :
The karta should first do, at least once, the aachamana while taking the pavitram from his acharya, preparatory to the start of the ritual. Likewise, the performance should end with an aachamana.

You know one has to do achamana twice at the time of discarding.

At the conclusion of the rite, he has remove the pavitram froebookm the finger and keep it over the right earlobe and do one Achaean. then he has to remove the pavitram from the ear, untie it and throw it in the niruriti (south-west) direction and do the aachamana again.

* Procedure for drinking water or do something different when the karma is on:
If the karta has to drink water or milk etc.,during the ritual, during the ceremony, he should not drink with the pavitram on. He should lodge it over the right earlobe and drink. The pavitram should not be kept elsewhere or handed over to anyone.

* Karpura Harathi & Pada prakshalana:
Similarly while accepting karpoora harathi during Deva puja, or doing paada prakshalana (a ritual of washing another person’s feet) the pavitram must be removed from the finger and placed above the right earlobe.

Ref: Sree Sarma Sastri's facebook post(Shared)