Samidadhana (समिदाधानम्)

It is prescribed by sastras that all the dvijas (the ‘twice-born,’ through Upanayana) of the first three varṇas are to be initiated into performing Samidhadhanam. A brahmachari is initiated into performing agnikaryam during Upanayana with prescribed procedures and is taught the relevant mantras to conduct it.

At the time of investing the yagnopaveeta, he is taught to perform daily aahuti in the fire called Praajapatyaagni check.

Brahmachari's should perform two activities - Sandhyavandanam and Samidhaadhanam in Agni everyday in the morning and evening. In Vishnupurana (3.9.3)

उभे संध्ये रविं भूप तथैवामग्निं समाहितः । उपतिष्ठेत्तदा कुर्य्याद्गुरोरप्यभिवागनम्॥

A bachelor should with devotion pray to surya and agni during both sandhyakaalas and also pay respects to his guru.

आग्निवेश्यगृह्यसूत्रम् ॥ Agnivesyagrhyasutra (1.1.4) outlines the process and mantras as follows

अथ सप्त पालाशीः समिध आर्द्रा अप्रच्छिन्नाग्राः प्रादेशमात्रा घृताक्ता अभ्याधापयति ।

Summary : Seven Palasa samidhas which do not have split ends (with their given dimension) along with ghee are to be offered.

अग्नये समिधमाहार्षं बृहते जातवेदसे । यथा त्वमग्ने समिधा समिध्यस एवं मां मेधया प्रज्ञया प्रजया पशुभिर्ब्रह्मवर्चसेनान्नाद्येन समेधय स्वाहा इत्येकाम् । अग्नये समिधौ इति द्वे । अग्नये समिध इति चतस्रः ।

In Asvalaayana Grhya sutras also the mantras to be recited during samidhaadhana are given

मन्त्रेण हैके अग्नये समिधमाहार्षं बृहते जातवेदसे तया त्वमग्ने वर्द्धस्व समिधा ब्रह्मणा वयं स्वाहेति ... (Asva. Grhy. Sutra 8.1.21.1)

Samidhaadhaana vidhi outline is given in Balabodhini Sangraham - 3 where the meanings of 13 mantras are explained. It is discontinued after marriage.