What would be the authentic source for Śrāddha vidhi?
Shraddha karma and other karmas like sandhyavandanam etc are described in
Ghriya Sutras -: The sutras for domestic ritiuals
Ghriya Sutras are decided according to your vedic shakha if you are a dwija.
Following is the Description of Various Ghriya Sutras and the Vedas from which they are derived.
Âśvalāyana-Gṛhyasūtra  Kausîtaki-Gṛhyasūtra (Bāṣkala śakha) Śāṅkhāyana-Gr̥hyasūtra 1
Gobhila-Gṛhyasūtra Khādira-Gṛhyasūtra (Drāhyāyana-Gṛhyasūtra) Jaiminiya-Gṛhyasūtra Kauthuma-Gṛhyasūtra
Baudhāyana-Gṛhyasūtra Hiraṇyakeśi-Gṛhyasūtra (Satyāsādha-Gṛhyasūtra)  Mānava-Gṛhyasūtra Bhāradvāja-Gṛhyasūtra Āpastamba-Gṛhyasūtra Āgniveśya-Gṛhyasūtra Vaikhānasa-Gṛhyasūtra Kāthaka-Gṛhyasūtra (Laugāksi-Gṛhyasūtra) Vārāha-Gṛhyasūtra Vādhûla-Gṛhyasūtra Kapisthala-Katha Gṛhyasūtra (unpublished)
If you are non-dwija person these don't apply to you. So just have shraddha (respect) towards your ancestors and give them food (via a brahmin or poor person).
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