The appropriate word that should be used for ‘something that is given by God’

Your original sentence is unclear to this extent: Do you mean that each spice should be put in separately (one spice at a time) or that they should all be mixed together into something like a curry powder and then added in equal fractional parts?

I'll assume the former (it's more rational for cooking for a couple of reasons).

You can say it this way:

Rather than adding all (of) the spices at the same time while cooking something, put them in one at a time at regular intervals, for example, every 2 minutes.

You can also say:

... put them in (separately) step by step, ..."

There's no need to clutter the sentence with unnecessary terms like "rounds" and "steps".

In general, you can say things are worn out.

What do you expect? I've been using them for the past two years. They're worn out!

For many objects, there are alternative words, but they're less general:

For your car example, I'd say dilapidated. This word works best for larger, more complex objects. A house or car can be dilapidated, but the phrases ^?dilapidated keys and ^?dilapidated headphones sound weird to me. You can also say a car is broken down, but only if it doesn't actually move.

For keys, I agree that rusty works, but only if they're actually rusty. If they're worn down, I'd say they're worn.

Sometimes you don't need a special word. I might complain about my broken old headphones. You could also say they're in a state of disrepair, but it doesn't sound very conversational to me.

^? This symbol means "This phrase is of dubious acceptability. I suggest you avoid this phrase."