You need to work your obliques, including the abs on the side. Bending side to side is a pretty good way to train your core and improve strength that applies to mountaineering, like picking up your really heavy pack, lifting a sled over a crevasse lip, digging out a tent platform.
The instructions for this one are pretty simple. In my case I like to use a barbell for the instability, but you could use something more compact and stable, like a kettlebell, dumbbell or weight plate. I grab the middle from a mid-thigh position off a rack, straighten my back and shoulders for good posture, then dip the bar up and down, lower and lift, pulling with my side muscles.
I avoid tilting my hip, bending my elbow and wrist, so that the force is applied better to the side ab muscles.
As for the other exercises, guard your back and joints, be careful, and really, you don’t need a lot of weight for this one. I usually use an empty bar (45 lb) for about 25 reps, but anything from 10 to 25 reps is good. These are tiny muscles overall, and don’t worry about bulking them up grotesquely – it would be really tough to do with a movement like this.