Well first things first you have to play as Isaac.
To get to Satan, you must kill moms heart; and If you killed moms' heart enough times or a devil room shows up, you go down into Sheol. Once you get to Sheol it's like all other floors just find your way to the boss room. When you get to the boss room, you will be fighting a 3 phase boss fight, You start by Fighting the Fallen, then you fight Satan, then you fight Satan's feet. After you kill the final phase just walk up to the chest and you get your Achievement.
As far as tips go... I can't give much, but if you've made it to Sheol, and have enough a good amount of hearts you should be fine. I beat Satan on my first try, Was it luck, mostly, but this game has a lot of RNG, So I hope you get as lucky as I did