Watch a lot of real NFL game tape. Start testing in Madden with teams you're familiar with. Track stats in Excel to make sure the results you're getting are within NFL averages; Pro Football Reference has league average data available here:
https://www.pro-football-reference.c.../NFL/index.htm
It is a lengthy process and you will likely change your sliders countless times either because of EA's patches or you noticing something in real life that you want to be reflected better in-game
If you have developped sliders in the past, definitely use those as a starting point. You don't have to start vanilla every year.
Read as much as you can about what each slider does before you change anything.
I could go on, and on, and on