1. Pour spaghetti noodles into the boiling water in a pot.

2. Put spaghetti noodles into the boiling water in a pot.
3. Add spaghetti noodles to the boiling water in a pot.
4. In step two, cut the red chillis into pieces on a chopping board.
[Are the sentences grammatical?]

I’d use 2, 3, and 4, but not 1.

Someone else may disagree!

Yes, all of the sentences are grammatical. They each provide a clear and concise instruction related to cooking spaghetti noodles.