Regarding the practice of Mantra & Tantra without the guidance of a Guru:

"Pustake likhitā vidyā yena sundari yapyate
Sidhir na jāyate tasya kalpakoti-ṣaṭair api.

O Beauteous one! He who does Japa of a Vidyā [Mantra] learnt from a book can never attain Siddhi even if he persists for countless millions of years."

Ṣaṭkarmadīpikā