To perform a barrel roll, bank sharply left or right, and gently pull back on the stick to maintain the roll. Hold sideways pressure on the stick as you roll inverted, then center it when you return to level flight.
During this move, the stick is essentially in the Left or right rear corner of the cockpit. The nose will draw a circle on the horlizon, instaed of rotating on a point. If you do a barrel roll perfectly, you won't lose any altitude