I find this question a little vague but I’ll try and answer as best as I can. However, a lot of other users have already summed up the process quite nicely.
My vagina felt strange before I started my period. Slight throbbing, aching, and a little bit of pain. It was mostly just discomfort, really. Nothing completely unbearable.
It was the stomach aches which got to me (I realise now it was pain in my uterus). The pain, for me, was horrific! I didn’t quite catch on that perhaps my period was looming, but when I told my mum about the pain she knew! I refused to accept that I was beginning my period, haha. I think my hormones were making me really moody. xD
As @genjgal said, in the future you will probably be able to predict when your period is coming because your body will let you know. I always get super hungry before my period, and extremely stubborn and emotional.