fighting game terminology, if you were using a piano you just slide from one key to the next. and doing a quarter circle is just, the second half of the half circle
[default controls, facing right] You tap D, S, D+S+attack. You want to hold S once you press it and then press forward and attack at once. When you're doing motion inputs, try to press the attack button at the same time as the last directional input. Hope that helps!
Fwiw this kind of input (fighting gamers call it the "dragon punch" after Ryu's move) is actually a little easier on keyboard than it is on gamepad! though it might help to move the keys to something more comfortable than WASD.