When you have successfully achieved a paṭibhāga-nimitta (a clear unchanging Nimitta achieved due to deep concentration) and can successfully pay attention to it almost without interruption and restlessness, then you can be said to have achieved jhāna. The Nimitta can be the object of ānāpāna, kasiṇa, a being that you're sending loving-kindness towards or an asubha. However, to really master in the first jhāna, one still needs to check and confirm thoroughly if the state of concentration is really jhāna or not by investigating the jhāna factors.
When you can focus your mind continuously on the object known as paṭibhāga-nimitta for an extended period, typically 2 to 3 hours, as a next step, you should attempt to identify the specific area within the heart where the bhavaṅga resides. This specific area near the heart is the heart base or the heart materiality: the base of consciousness (citta). The bhavaṅga is represented in the commentaries as the mind door (manodvāra), and is described to be bright, clear and luminous. Why do we discern at the mind door? This is because the paṭibhāga-nimitta is a mental object (dhammārammaṇa object) and can be experienced through the mind door only. Therefore, to discern the jhāna factors, one must first practice paying attention to the bhavaṅga (or mind door). It should be done with a careful attention and consistently but for around 3-5 seconds each time only. The attention to the mind door must also not happen all the time, preferably only once every hour or when you have achieved deepest level of concentration.
Eventually, you will be able to perceive the mind's gateway (bhavaṅga) skilfully and clearly. If you can do so, you should try to see if you can see the nimitta that you're focusing is reflecting on the mind door. For instance, if you are practicing red kasiṇa, you will see the red kasiṇa object reflecting on the mind door. The more skilful you get with the discernment, the more clearly you can perceive the paṭibhāga-nimitta in the mind door. Only then, you should start to discern the five jhāna factors individually, and only once per sitting. With continued practice, you will eventually be able to perceive all five factors simultaneously within a sitting. These jhāna factors are:
- Direct application (vitakka): directing and placing the mind on the paṭibhāga-nimitta.
- Sustained application (vicāra): maintaining the mind on the paṭibhāga-nimitta.
- Joy (pīti): experiencing a sense of delight for the paṭibhāga-nimitta.
- Bliss (sukha): feeling a state of happiness from focusing on the paṭibhāga-nimitta.
- One-pointedness (ekaggatā): undistracted focus on the paṭibhāga-nimitta.
The collective term for these five jhāna factors is simply referred to as "jhāna." As you commence your practice of jhāna, it is advised to immerse in the jhāna for an extended duration as your main objective, and refrain from investigating the jhāna factors frequently and unnecessarily. Doing so can impact the quality of concentration negatively.
It is recommended that a practitioner who is starting off this method of discernment to seek close guidance from the meditation teacher and not to try this on your own.
May you realise the four noble truths in this life.