first time grow

the girls have really come on, well 3 of them, albeit they're different heights, think I over did it with the nutrients as the ends of the leaves are brown and curling upwards, only the very tip, someone did mention it could be lack of potassium? not sure how id remedy that? Anyways for last 3 waterings ive just given them bottled water, nothing else, think the tallest one is ready after ive finished flushing (am I right in thinking I need to flush for around 14 days or 14 flushes?) I am really proud of my first grow and will defo be doing it again, had a lot of help and advice on here which is fantastic, good solid knowledge, thank you. please answer my question re cutting and drying, 😀🙏