Theres a lot of ways to do it, most Gms though will just roll for the player, especially in 1on1s, by using a program or something, i like to use,
http://invisiblecastle.com/roller/ and
https://www.wizards.com/d20modern/d20mdice/dice.htm Then you have players have stats to modify their rolls either giving plus or minus to the end result. Most of the time as well, 11 and above is a success, with 20 being a crit success, and then on the flip side 10 and lower is a fail with 1 being a crit fail.
I usually roll first then write out the scene based on the roll, depending on the complexity i'll roll mostly "attacks" or "check" and not roll defense unless it's stated by the player they want to defend or a defense is just inate, like a lust spell attack, i'd roll the player's Wis or will and if they fail they get hit by it. In more complex games i could roll the Imp or demon's attack but more often then not i dont.