Curtis explains it best-

The most important part of doing a loop or a roll is the straight flight into these maneuvers.

So practice straight and level flybys at about 3/4 power, with special emphasis on keeping the rudder/tail straight.

Then the actual loop is just entering in straight and level flight at 3/4 power. Pulling back on the elevator and slowly going to full power. Continuing to hold the elevator, slowly reduce the power to about half stick as you go past the top of the loop (do not reduce power before the top of the loop, if you do the heli will tend to stop at the top inverted). As you come out at the bottom of the loop slowly let off the elevator and increase back up to 3/4 throttle. This is a very basic beginner loop.

As for the roll, enter at 3/4 throttle or a little less. You do not need much speed to just get through a roll. Then give about 3/4 (or up to full) aileron. As the heli gets inverted keep holding aileron and go to just below center stick on the collective (about -2 degrees). Continue holding the aileron and the negative until you are all the way right side up and then return to 3/4 collective and let off the aileron. That is the basic roll.