Scriptural basis for the Shodashopchar Puja?

Question

What is the earliest reference to Shodashopchar Puja in scriptures?

Which scriptures gives the exact details of its procedure?

Answer 1

The whole concept is sixteen upacharas is explained in Skanda Purana.

Indra worshipped Devi Lakshmi with these 16 upacharas.

Indra worshipped devotedly with sixteen upacāras (articles offered). Every upacāra (article) was offered with the repetition of mantra. All the things were very excellent, right and commendable. “O Mahā Lakṣmī!

Devi Bhagwatam (9:42)

Answer 2

I don't know what does "earliest references" mean. Because of the cyclic nature of time in Hinduism theology (cf. Srimada Bhagvad Geeta 2.13) every text in of Hinduism has an eternal antiquity.

References to shodashopachara puja are found in numerous texts like Puranas and tantras, etc.

One such reference is found in the Shiva Purana.

Shiva Purana mentions about the shodashopachara (षोडशोपचार) form of worship.

Shiva Purana Verse 1.7.25-29

अथवा चरलिंगं तु षोडशैरुपचारकैः ॥ २५ ॥
पूजयेच्च यथान्यायं क्रमाच्छिवपदप्रदम् । आवाहनं चासनं च अर्घ्यं पाद्यं तथैव च ॥ २६ ॥
तदंगाचमनं चैव स्नानमभ्यंगपूर्वकम् । वस्त्रं गंधं तथा पुष्पं धूपं दीपं निवेदनम् ॥ २७ ॥
नीराजनं च तांबूलं नमस्कारो विसर्जनम् । अथवाऽर्घ्यादिकं कृत्वा नैवेद्यां तं यथाविधि ॥ २८ ॥
अथाभिषेकं नैवेद्यं नमस्कारं च तर्पणम् । यथाशक्ति सदाकुर्यात्क्रमाच्छिवपदप्रदम् ॥ २९ ॥

The sixteen types of service are:—invocation (Āvāhana); offering the seat (Āsana); water offering (Arghya); washing of the feet (Pādya); water for rinsing the mouth as a mystical rite (Ācamana); oil bath (Abhyaṅga snāna); offering of cloth (Vastra); Scents (Gandha); flowers (Puṣpa); incense (Dhūpa); lamps (Dīpa); food offering (Nivedana); waving of lights (Nīrājana); betel leaves (Tāmbūla); obeisance (Namaskāra); and mystical discharge and conclusion (Visarjana)

English Translation by J.L. Shastri.