Another method:
Accelerate with full throttle until you reach V1. Activate airspeed hold (Alt + S) Pitch up to 10 degrees. At wheels up, raise landing gear. Once landing gear up, turn to your ascent heading. Once you are turned to your heading, disengage airspeed hold with spacebar, accelerate to full throttle again, and pitch up to 70 degrees. Maintain this pitch until about 20-24 km, then level off. Accelerate to Mach 3.5, open the scram doors, engage the scrams. Once the scrams are producing enough thrust, slowly back down the main engines. Fly with scrams as long as possible, usually until around 60-65 km. Use the attitude hold autopilot to control your pitch and maintain a vertical speed of about +100 m/s during the scram burn. I just let the scrams shut themselves off automatically once they get too hot. Remember to close scram doors, then engage main engines at full again, raise ApA to desired altitude, coast to ApA, then circularize your orbit.
That's how I do it anyway. Works well for me.