How can I become a prejac that cums at just getting touched

I am doing a 2 week denial challenge of no cumming how can I get ready to prejac train i have only been using 2 finger every time I masturbate. I already cum in less than 2 minutes if I use my hand but i want to be less than 30s. I heard clench training could work does anyone have advice on the best way to ruin my stamina and how often I should train