This page gives a reasonably easy to follow and a scriptural method by which one , who does not have a Guru Diksha but desperately in need of one, can make Lord Dakshinamurthy his/her Guru.
As much as possible, one must try and obtain a guru, to guide with
mantras and properly initiate the devotee or sādhakā. But often, we
are faced with a situation, where we cannot find an appropriate guru
to guide us. In such situations, one can go ahead and initiate
themselves. The procedure to initiate, as well as guidance from the
śāstras or scriptures, is provided below.
The Prayoga needs to be done on the Trayodashi Krishna Paksha. The Sanskrit verses quoted from Agama Rahasyam 4th Chapter say that it has to be done in a temple of Lord Shiva/Dakshinamurty. But the author says that if that is not possible then the same procedure can be followed in one's own house too in front of an image of Lord Dakshinamurty.
gurorabhāve mantrāṇāṃ grahaṇa krama mucyate।
kṛṣṇapakṣe trayodaśyāṃ dakṣiṇāmūrti sannidhe॥
गुरोरभावे मन्त्राणां ग्रहण क्रम मुच्यते।
कृष्णपक्षे त्रयोदश्यां दक्षिणामूर्ति सन्निधे॥
labhitvā rājate pātre tālapatre'dhavāpunaḥ।
mantraṃ tatstaṃḍile stāpya pūjaitvā maheśvaram॥
लभित्वा राजते पात्रे तालपत्रेऽधवापुनः।
मन्त्रं तत्स्तंडिले स्ताप्य पूजैत्वा महेश्वरम्॥
pāyasādi nivedyaṃ ca kṛtvā taṃpraṇi patyaca।
śatākṛtaḥ paṭhenmantraṃ dakṣiṇāmūrti sannidhau॥
sarveṣāṃcaiva mantrāṇāmevaṃ grahāṇa miṣyate॥
पायसादि निवेद्यं च कृत्वा तंप्रणि पत्यच।
शताकृतः पठेन्मन्त्रं दक्षिणामूर्ति सन्निधौ॥
सर्वेषांचैव मन्त्राणामेवं ग्रहाण मिष्यते॥
āgama rahasyaṃ caturtha paṭhale (śloka 42-44)
आगम रहस्यं चतुर्थ पठले (श्लोक ४२-४४)
The first line itself "gurorabhāve mantrāṇāṃ grahaṇa krama mucyate" means "in the absence of the Guru the following is the process for getting self-initiated into a Mantra".
The above ślokas state that anyone interested in initiating himself,
due to the non-availability of a proper guru, may do so on the 13th
lunar day (trayodaśi) of the dark phase of the lunar cycle
(kṛṣṇapakṣa). The devotee must visit a śiva temple and pray to Lord
dakṣiṇāmūrti, who’s placed in the southern direction within the same
temple. If the Lord dakṣiṇāmūrti idol is not present, then one can
seek the idol of Lord śiva or nandi (the bull facing the śiva liṇga)
and on a silver sheet, or if not available a brass sheet, or further a
palm leaf or just a simple paper, write the mantra related to the
mantra japa, that one wishes to perform. Place the sheet in front of
the aforementioned idol and sincerely perform the puja or prayer to
seek the guidance from the Lord, requesting Him to be the guru and
praying for the success of the mantra japa. If a temple is not
accessible, then one can pray to the idol or image of Lord
IMO, this is reasonably very easy to follow. And, one can try it when one can not find the right Guru.