If you were right handed and firing a rifle, you would stand with your feet shoulder length apart, left elbow comfortably at your ribs, left hand cradling the rifle, right had holding the gunstock, index finger on the trigger, ready to squeeze it, and breathing normally.

Similar with a camera. I stand with my left shoulder oriented towards the subject, finger on the shutter and breathing normally. I find that when I pause at the end of an exhale I am most relaxed and I get better results. Breathing is very important. Never ever hold your breath.

This guide which applied to shooting with a rifle explains the whole thing about breathing. Bottom line, when you hold your breath muscles tighten and your heart rate changes making you less stable.

http://blog.beretta.com/bid/308231/Don-t-Forget-to-Breathe-Four-Breathing-Methods-for-Shooters

The first shot below was hand held with a 150-600 Sigma Sport and a D800 about 18 ft from the subject, 600mm, @ 1/25 second with optical stabilization on. Camera and lens total weight is around 8.5 lbs.

The second shot was at a much easier 1/125 second, also near the minimum focus distance. Typically I find shutter speeds between 1/125 and 1/640 are aided tremendously by stabilization. Generally I turn stabilization off for shutter speeds shorter than 1/800 sec, or if I am panning a flying bird.

Since 2016 when I purchased the lens, it has only been on a tripod once, for a moon shot. That's about 11,000 images shot with that lens and only a tiny handful on a tripod. I shoot a lot of wildlife so it is my walk around lens. I don't use a monopod, string, bean bag or anything else.

I suggest you try to perfect your hand holding technique before trying anything else.