Bound Side Angle Pose
In Bound Side Angle Pose you wrap your arms around your thigh and torso and grab a wrist behind your back to create the bind.
Binding Difficulty Level
On a scale of difficulty for binding poses this pose is slightly easier to bind in than the Modified Marichyasana B pose because you only bind the thigh and not the thigh and shin together as in that pose.
However it is harder in that it requires thigh strength to stay standing. And that's actually one of the benefits of this pose, it strengthens the legs.
Note that you don't have to bind in order to get the leg strengthening effect. You could do side angle pose with the arms lifted and strengthen the thighs that way.
Getting Ready to Bind
Start in a normal non-binding version of side angle but with your bottom arm to the inside of the thigh.
Make your bent knee leg strong so that you can easily support your body with your hand off of the floor.
So that you can bind easily in this pose you need to get your shoulder below (or at least at) the level of the bottom of your thigh. To that end, bend your bottom elbow and reach it to the floor.
If you can't get your elbow close to the floor, your shoulder is probably nowhere near where it needs to be in relation to your thigh. So spend some time in this pose. You can put your hand on the floor to rest, then focus on reaching your elbow to the floor.
One you can do that you can work on the next step.
Working Towards Binding In Side Angle
With shoulder at, or below, the level of your thigh, reach your bottom arm rewards, beneath the leg.
You can hold here, just getting used to this position (try it on both sides) then from there internally rotate your arm.
Internally rotating the arm should cause the point of your elbow to point forwards (in the same direction as the toes of your bent-knee foot.)
If possible, increase the internal rotation so that the point of your elbow points more towards the floor. Then bend your elbow.
Your hand should end up close to or at your lower back. From here you can reach your other hand behind you and grab it's wrist (or at the very least, clasp fingers).
Which Hand Do You Grip With?
Note that changing which hand does the gripping (if you are grabbing a wrist) you can change the way the pose feels. I'll suggest here that neither option is wrong or right.
If you grab the top wrist you can try to straighten the top elbow. You may find that this causes you to roll your top shoulder back, opening that side of your chest.
If you grab the bottom wrist you can try to straighten the bottom elbow. You may find that this causes your arm to press up against the bottom of your leg. This in turn helps to pull your torso lower.
You could try interlacing your fingers and then straighten both elbows. Then you can do a dwikonasana action and reach the arms back behind your back. (Dwikonasana is the arm position use in prasaritta padotanasana c).
Another possibility is that you count the breaths you are in this pose and try to last as long as possible before your thigh gives out.