For seated side bend you can start with your legs spread apart. You can keep both knees straight, or bend one knee and fold it to the inside of the thigh (janu sirsasana position) or the outside of the thigh (hero position.)
Turn slightly away from the side you are bending towards. Then lengthen your spine and bend your spine towards one leg.
Depending on your flexibility (or lack there of) you can place your hand in front of the leg or behind it.
Generally, for people who are less flexible, placing the hand behind the leg makes this seated yoga pose a little bit more comfortable.
I generally like to start with the arms relaxed and on the floor. I then focus on bending the spine to the side.
In this version of the side bend the shin of the bent knee leg is folded to the outside of the thigh (like in half hero pose or triang muka eka pada paschimotanasana). With the bent knee shin folded to the outside of the thigh, it is easy for the "bent knee side" of the pelvis to lift off of the floor.
As a result this version of the pose can be used to stretch the hamstring of the straight leg as well as the "bent-knee" side of the waist and ribcage.
Note that another way to do seated side bend is to have the bent knee shin folded to the inside of the thigh so that the heel of that foot is in front of the groin.
With this position you can focus on pushing both your knee and the "bent-knee" side of the pelvis down to the floor.
This foot position focuses the stretch more on the side of the waist and ribcage, stretching the obliques and intercostals.
The intercostals are the muscles that are located between each set of ribs. They help to expand, contract and change the shape of the ribcage.
With your lower hand on the floor in seated side bend, you can use that arm to help support the weight of your body. This may make it easier for your obliques, intercostals and hamstrings to relax.
You can then bend your elbow to lower your body under control, gradually stretching the side of your waist and ribcage.
Another option is to lift your arms off of the floor. In this case you add weight to the pose. The further you reach one or both arms to the side the more weight you add to the pose. If you slightly lift your head and lengthen your neck you may find that even that simple action adds a little bit more weight making it easier to stretch.
You can also work towards grabbing your foot (if you can't already do so.) Once you do have your foot in your hands you can use your arms to twist and turn your ribcage so that your chest stays open to the front. You can also use them to help lengthen your torso towards your foot.
To make grabbing your foot easier in seated side bend you can lift the "bent-knee" side of your pelvis off of the floor. This action will lessen the side stretch but deepen the stretch to your hamstrings.
To the half hero version of seated side bend, open both legs out to the side. Bend your right knee and fold the shin to the outside of your foot.
If you need to, bend the knee of the straight leg. You can always straighten it as your hamstrings and side body lengthen.
Whether your left knee is straight or slightly bent, keep it pointing upwards.
To prevent stress on your right knee, keep the top of your bent knee foot facing down.
If you prefer to sit with the inside edge of this foot on the floor make sure that there is no "pulling" sensation on the inside of your right knee. Try to roll your right thigh inwards as much as you can.
Sit up tall and turn slightly towards your bent knee leg. (In this case, since your right knee is bent, turn towards the right.) As you sit up taller use your weight to press both sitting bones into the floor.
If you have knee problems you might need raise yourself up by sitting on a block.
When you are ready, tilt your body to the right. Place your left hand inside your leg (as I am doing in the picture) or if it suits you better, you can also place your hand to the outside of the leg.
In either case use your arm to support the weight of your body. Then to help your ribcage move down, slowly bend your elbow.
While lowering your ribcage focus on relaxing the right side of your waist and on keeping it relaxed.
When you are ready, slowly relax your left arm completely so that you are no longer supporting your ribcage. Then lift your hand off of the floor and add weight to the stretch by slowly reaching your hand towards your foot.
To add more weight to seated side bend, reach your right hand up and over your head to the left.
Lengthen your spine towards your foot and at the same time slowly allow your ribcage to sink towards your thigh.
You can place your left hand on the floor and reach it forwards to pull the left side of your ribcage forwards.
At the same time roll the right side of your ribcage back, as you did while you where twisting. You can also press your elbow or shoulder against the inside of your thigh to help you deepen the twist.
The idea isn't to deepen the twist, but to make the side stretch as deep as you can.
Keep your neck long and your head slightly lifted to add more weight to the pose.
As you get deeper into this pose your right hip will come up off of the floor. You can use this action to stretch the hamstring of your left leg. The higher your right hip lifts the deeper you will stretch your left hamstring. However in the process you'll lose some of your side stretch.
If possible you can grab on to your foot in seated side bend. Grab with one or both hands. You can then use yours hands to help pull your ribcage towards your foot (lengthening your spine in the process).
If you are having difficulty grabbing your foot, you might find it easier to lift the right side of your pelvis upwards. You can also roll the right side of your chest forwards so that you can grab your foot. From there, relax your right shoulder, lengthen your neck and try to peek upwards from beneath your arm. Slowly work at rolling the right side of your chest "back" so that your chest is open.
If you've grabbed onto your foot you can deepen the stretch to the side of your body and even to your shoulder by trying to push your right hip back down again.
To sit up you can try slowly sitting up with your arms still above your head. This will help to strengthen and activate the muscles you've been stretching.
If you do choose to do this, start slowly and smoothly to prevent injury. If you can't do it then lower your arms first and then sit up.
While holding seated side bend (or moving in or out of it) you can focus on using your inhales to lengthen your spine. You can focus on relaxing your waist and allowing your ribcage to sink down while exhaling.
You may find it easier to stretch your hamstrings if you focus on reaching your knees away from your your hip socket. This may also help you to deepen your side bend.
When grabbing on to your foot focus on relaxing the shoulder of your upper arm.
If the side of your waist or your hamstrings are so tight that you can't use gravity to help you sink down, sit up higher, or a short stool or some yoga blocks so that you can bend to the side.
Also try bending your "straight leg" knee.