Diksa-guru is Krsna Himself that manifested in this material world; a manifestation of Baladeva Prabhu or Nityananda Prabhu. There are many types of guru: diksa-guru, siksa-guru, vartma-pradarsaka-guru, mahanta-guru. Actually, mahanta-guru is Krsna Himself. As like Lord Caitanya Mahaprabhu, He is also acting the role of guru. 

Caitanya Mahaprabhu is Krsna, but He is taking the mood of bhakta, bhakta-bhava, and as a bhakta, He is acting the role of guru; so Caitanya Mahaprabhu is also acting the role of mahanta-guru. And at the same time, Caitanya Mahaprabhu is Krsna, so He resides in the heart of all the living entities; in the heart of the devotees. 

So, Caitanya Mahaprabhu is acting four roles: Krsna, devotee, caitya-guru and mahanta-guru.

The Paramatma, Supersoul, resides in the heart of all the living entities and His duty is just to witness; what the living entities are doing, He is just watching it. When the living entity’s sukrti is matured and accumulated and he has surrendered unto the lotus feet of guru, then caitya-guru who resides in the heart, Paramatma, gives us inspiration on how to develop our bhajana and sadhana. This is the process.

In Srimad Bhagavad-gita, Krsna says: “I am also Paramatma, the Supersoul; I reside in the heart of all the living entities”. And the Supersoul is also called mahanta-guru.
teṣāṁ satata-yuktānāṁ
bhajatāṁ prīti-pūrvakam
dadāmi buddhi-yogaṁ taṁ
yena mām upayānti te
[“To those who are constantly devoted to serving Me with love, I give the understanding by which they can come to Me.”]

“Dadami buddhi-yogam”, when your sukrti is matured and accumulated, then Krsna, as the Paramatma (Supersoul) who resides in the heart of devotee, will give inspiration in your heart on how to develop your bhajana and sadhana. 

So first, this guru is called Paramatma (Supersoul), caitya-guru. Then, caitya-guru, Paramatma, gives inspiration to you on how to go to the path of bhakti. Sometimes caitya-guru will make you directly meet with diksa-guru or siksa-guru and sometimes He will also arrange on how you will meet with Krsna, as vartma-pradarsaka-guru, one who leads you to the path of bhakti. This is the process. 

When you take diksa and chant the holy names, slowly, slowly, the realization will come. First comes sambhanda-jnana, knowledge of relationship. Because when you are chanting this gayatri-mantra, it makes the sambandha-jnana, the knowledge of relationship with guru and Krsna. And this sambandha-jnana starts in the stage of sraddha. “Sraddha, sadhu-sanga, bhajana-kriya, anartha-nivritti, nistha, ruci, asakti”... Diksa means:
divyaṁ jñānaṁ yato dadyāt 
kuryāt pāpasya saṅkṣayam
tasmāt dīkṣeti sā proktā 
deśikais tattva-kovidaiḥ
(Caitanya-caritamrta Madhya 15.108)
[“That process which bestows transcendental knowledge (divya-jnana) and destroys (ksa) sinful desires for sense gratification is called diksa by learned authorities in the absolute truth.”]

First, “divya-jnana”, transcendental knowledge will come. Then, simultaneously, all your unwanted things, anarthas, are removed from your heart. So there are two types of diksa: loukik-diksa (worldly diksa) and paramartik-diksa (spiritual initiation). That means, when your sukrti is matured and accumulated, your inspiration will come, “I have to take diksa from guru and chant gayatri-mantra, kama-gayatri, gopala-gayatri-mantra”... 

There are two types of paramartik-diksa: first is anustanik-diksa, the formality; approaching in front of guru, offering flowers, fruits, some daksina; that is called ‘upanayana’, which means ‘offering’, the significance of your heart and mind. Then guru gives you a paper with the mantras and tell you to chant them. 

Then there are two types of how to develop your diksa-mantras in the proper way: vaidhi-diksa and raganuga-diksa. 

Vaidhi-diksa means following all the rules and regulations of chanting the diksa-mantras, which guru explains to you: wake up early in the morning during brahma-muhurta, take bath, put tilaka, kanti-mala, do acamana, “idam acamaniam om kesavaya namah, idam acamaniam om narayanayana namah, idam acamanyam om madhavaya namah. om sri visnu sri visnu sri visnu”. In the morning you should be facing east and in the evening, facing north. There is also a mudra and you count 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. So these are the rules and regulations of vaidhi-diksa. 

And raganuga-diksa is actually bhagavati-diksa, starting when you listen harikatha from any bonafide sadhu and guru, then one kind of greed comes in your heart, especially one greed will come: “I want to become the maidservant of Srimati Radhika”. This type of greed will come when you listen harikatha. 

Actually, all diksa process starts from sraddha, so sraddha, sadhu-sanga, bhajana-kriya, anartha-nivritti, nistha, ruci, asakti and bhava. When sadhaka will come in the stage of bhava-dasa, then all his diksa will be completed. But by taking initiation from guru, in one day, diksa is not completed; it is on the process.
When you registered your name at school; that time you were called ‘student’. The student goes to the 1st class, 2nd class, 5th class... But now you are also students! And even one baby from the 1st class is a student; and one who is on the 10th class is also a student, but they are in different stages. This is the process, how to step by step develop your spiritual life. So when sadhaka will come in the stage of bhava, he completely realizes his transcendental constitucional form and with this form he serves his ista-deva, the divine couple Radha and Krsna.