The substrate is the material the mycelium eats. For oyster in India it is usually wheat or paddy straw; DMR notes most such substrates hold 0.5-0.8 per cent nitrogen, enough without additives. Use fresh, dry straw from the current season, stored under cover; old or rain-wet straw brings ink caps and moulds.
Chop so it packs evenly. The DMR folder machine-cuts hard residues to about 2-3 cm; the 2011 manual chops 5-10 cm for hot water; TNAU uses 5 cm. DMR allows wheat or rice bran as a supplement but warns it raises bag temperature by 2-3 C or more, so not in summer.