Regardless of the hardness and the fish that are finally bought, the tank needs to be cycled. Unless the tank is to be planted like a jungle with fast growing plants, it is better to cycle the tank first, then add plants. Rocks and substrate can be put in at the start.
This is how to cycle a tank
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
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With a tank for just a betta (assuming the hardness is OK for a betta) you wouldn't need to use as much ammonia. I cycled a 6.6 gallon tank for a betta using 1 ppm ammonia and that was plenty.
You mentioned using fish food to provide the ammonia - this is possible, but the food has to rot to make ammonia and you cannot know how much ammonia has actually been made. if you can get some ammonia, or Dr Tim's ammonium chloride, that is a more reliable way to cycle a tank.
I don't understand what the pack means when it says to let the water age, unless it's a translation from another language and it means to cycle the tank.