As you inhale, begin to draw the standing leg thigh behind you (which will straighten your front leg) as you lift the back leg parallel to the ground, and draw your torso down parallel to the ground. Depending on your level of stability, your arms can rest in prayer position on your chest, be held out to your sides, or stretch your arms forward as you press back through the lifted heel.
To avoid hyper-extension of the standing leg knee, press the calf forward into the shin as you firm the thigh behind you.
To practice this pose, you may choose to place a chair in front of you. Lightly press your fingertips into the back of the chair as you lift the back leg and straighten the standing leg.
Additionally, you can practice with a partner. Have your partner stand in front of you with their palms held out. As you move from Warrior 1 to 3, place your forearms on your partners palms. As your partner provides gentle resistance, it will be a bit easier to maintain your balance.