First I will talk about Mantras that don't need Diksha. These are Mantras which have special importance in our religion.
1. Mantra gotten in dreams
2. The Mantra whose Diksha is gained from a female Guru
3. The Mantra which has more than 20 letters
4. The Mantra which has more than 3000 letters
5. Vedic Mantras
6. Hans Mantra
7. 8 lettered Mantras
8. 5 letter Mantras
9. Mantra with one Beej, 2 Beej or 3 Beej Mantras in it.
10. Krishna Mantras
11. Gopal Mantras
12. A 3 lettered Mantra also does not require Deeksha.
All the above mentioned Mantras don't need Diksha and can be chanted by anyone.
Now I will share about Mantra favourability. Not every Mantra is suitable for a person. The knowledge I am sharing here is given in few books but I am still sharing so that one does not have to buy numerous books about our religion.
There are various Chakra one needs to see before starting his "Mantra Sadhana".
First and foremost important is Kulaakul Chakra. This Chakra has to followed for every god or goddess except the following- the 10 Mahavidya, the Shri Vidya Goddesses namely the 3 Tripur Sundaris, Svapnaavati Madhumati.
Other than this, Rakt Kaali, Mahishaasurmardini, Durga and Pratyangira also do not come under this Chakra.
Procedure to see Kulaakul Chakra:
1. The procedure is very simple. One must first look at the first letter of his name and then look at the first letter of the God's name. If both of them fall in the same column, it means that the God is of your Kul only and his Mantra is extremely favorable.
A special and important thing one must not forget is that every God has various forms. And every form may not be favorable. For example, Kali has more than 10 forms and every form may not be favorable. Hence, in such a case, the Sadhak must first look at the Viniyog of the Mantra he wants to do Sadhana of. Then, in the Viniyog, he must look at the name of the God given in it and then utilize the Kulaakul Chakra.
The Mantra Sadhana cannot be done with a Mantra which doesn't have Viniyog. So if you are unable to find Viniyog of a Mantra, you cannot do the Sadhana.
Other than what I have mentioned in 1st point, the following Varg should also be kept in mind:
1.Jal + Bhumi
2. Agni + Vaayu
Enemy: Vaayu + Bhumi
Agni + Bhumi
The Aakash Varg is favourable Varg for all.
Remember that the Vidyas I had mentioned in the starting dont require Kulaakul Chakra introspection but you still need to check other Chakras I am mentioning ahead.
After checking Kulaakul Chakra, he must look at Raashi Chakra:
1. Mesh Raashi(Aries Moon Sign) - अ, आ, इ, ई।
2. Vrushabh Rashi - उ , ऊ, ऋ
3. Mithun - ऋृ, लृ, लृ
4. Kark - ए, ऐ
5. Sinh- ओ, औ
6. Kanya - अं, अः , श, ष, स, ह, ल, क्ष
7. Tula - क, ख, ग, घ, ङ
8. Vrishchik- च, छ, ज, झ, ञ
9. Dhanu- ट, ठ, ड, ढ, ण
10. Makar - त, थ, द, ध, न
11.kumbh- प, फ, ब, भ, म
12. Meen- य,र, ल,व
For Rashi Chakra, one must look at his Rashi and then the Rashi of the God whom you found in the Viniyog.
1. If the God's Rashi is 8th or 12th from your Rashi, then the Mantra is not favourable.
2. If the God's Rashi is 1st, 5th, or 9th from your rashi, it is favourable Mantra Sadhana
3. If the Mantra is 2nd, 6th or 10th from your Rashi, the Mantra will give benefits but not complete benefits. It may only give limited benefits.
4. If the Rashi of the God is 3rd,7th and 11th from your Rashi, then the Mantra is very beneficial and it's chanting will benefit not only you but also your whole family.
5. If the God's Rashi it is 8th or 12th from your Rashi, doing that particular Mantra Sadhana will bring your death
The Sadhak must keep one rule in mind:
1. The counting of Rashi should be started from your Rashi itself and not the Rashi ahead of yours.
NAKSHATRA AND GANA CHAKRA:
This Chakra helps to know whether the Gana and the Nakshatra of the Sadhak and God is favourable or no. First, he must look at his own Gana in the table and then God's Gana in the table. After Gana(गण) Chakra, one must look at Nakshatra Chakra.
I have attached a picture of the Nakshatra and Gana Table. After referring to the Table, he must look at the results given below:
FOR GANA CHAKRA:-
1] If Sadhak's Gana is Maanav or Dev, then the same will be beneficial. Rakshas Gana will be harmful.
2. If the Sadhak's Gana is Rakshas, then only Mantras of a Rakshas Gana will be favourable. Dev and Manav Gana Mantras well be harmful.
FOR NAKSHATRA CHAKRA-
A. If the Mantra Nakshatra is 1,2,4,6,8,9,10,11,13,15,17,18 or 19 from the Sadhak's Nakshatra then the Mantra is favourable and Sadhak can get success.
B. If it is 3,5,7,12,14,16,21,23,25 then Sadhak will end up destroying himself or even die.
Indeed Mantra Sadhana is really long and complicated.
The things that I have shared tool now and going to share in future are irreplaceable part of a Sadhana. The way air is important for a human, the same way all Sadhana rules are necessary and inevitable. Such knowledge is not known to the general public because it only shared by gurus to their disciples. Hence it is said that having a Guru is utmost necessity for a Sadhak.
This is all for the first part.