The following are stretches you can do for each muscle group. Perform these stretches after your workout when your muscles are already warm and pumped. Hold the stretched position for 20-30 seconds; after a few sessions (once you've acclimated a bit), increase the duration to 60 seconds.
Chest. Grab a pair of dumbbells that you can perform 12-15 reps with on a dumbbell bench press, and get into the bottom position of the exercise (your hands near the outside of your chest).
Back. Hang from a chinup bar in the bottom position of a pullup, palms facing away from you.
Biceps. Set an incline bench to a 45-60-degree angle and sit on it with a dumbbell in each hand. Allow your arms to hang.
Triceps. Hold the end of a towel with one hand and grab the other end behind your back. Pull down with the hand behind your back so that your opposite elbow points upward and that arm's triceps is stretched. You should look like you're in position to do a one-arm overhead triceps extension.
Quads. Rest one foot on a bench set to a low incline, and bend the opposite knee so that you go into a split squat position. You should feel the stretch in your rear thigh and that leg's hip.
Hamstrings. Rest one heel on a bench, step, or other surface that's three or four feet off the floor. Bend forward and try to kiss your knee.
You can also stretch on a non-training day. Do some kind of warm up first (jump rope, light jogging), and then stretch gently for 15-30 minutes. Do not try to stretch as deeply as you do on your training days.