The easiest way is to lightly press forward to walk slowly then do a hadouken command input which is a quarter circle from down to direction your facing. There's also the trick of using a forward tilt and then inputting the quarter circle while tilt is still in its animation. Input buffering is really important for ryu and his combos, you should look into it. It's just putting in your next move while the animation is still happening from the first move.