We practice wholesome mind and mind factors on one meditation object, not nimitta, however nimitta is an effect of more power mind. When wholesome mind begin
to concentrate stably more and more gradually, the Nimitta is going to appear brighter and brighter little by little gradually, too. At first, it would be like a very transparent gray could, pin, etc., after it would be like a bright star, or sun, or flower, etc. And when it is a PatibhagaNimitta, it is super beautiful bright and stable like your stable mind. This may use 1-3 hours per sitting, 12h a day everyday for a year in monk, but someone may use just few months. It depends on one's past Pāramī. However, Nimitta appearing or not is not important because when the years of the concentration gone by, you can see the developing stable, claim, fast thinking which being better for complex insight meditation. That's the point to keep the concentration meditation for years to the the of life. I do it for 1-2 hours a day, 5 times a week for 2 month there are just unstable transparent cloud and some spectrum color appear to me. I try hard to care not of them because I lose the meditation when I lose focus from the meditation's object to Nimitta, broken meditation.
So, the only way to do is relax your mind and body, don't excite of whatever happen. Just go back to the meditation object, breathe, etc. We will focus on nimitta only when the nimitta is stable brighter and brighter enough. Then it is going to overlay the meditation object. When this overlaying nimitta is stable enough, we focuses on Nimitta instead.
This maybe require long time for layman. So it's okay if you want to meditate or listen the insight meditation if you worry you will miss the insight meditation. However, my experience tell me "I can't enlighten Nibbāna, although I meditate the insight meditation, if my 5 hindrances still often arise, no citta-visuddhi. When I realized this, I trow my time on the concentration meditation more and more, thenmy insight meditation is better than I have been before.
If I could go back, I will start the concentration meditation as early as I can and keep it to the end of my life, **either getting Nimitta or not because my Sutta and Abhidhamma study are easier after I've focused on the concentration meditatio. Another my insight meditation and emotion controll are better, too.
"Nimitta"'s references in DN.SubhaSutta to overlay the "(Kāma-)Nimitta and it's detail" in 6 SatiSanvara, "Escape Kāma and Escape Unwholesome" in 1st Jhāna, and "Pariyodāta (for Ānenjā)" in 4th Jhāna and 8 Vijjā: