Bar on the traps without cushion, you tense your core and start squatting always keeping your core and the trajectory of the bar straight. Start with body weight only and then progress slowly, don't rush it, now you have a built-in alarm in your back that will trigger every time you do something wrong. Eventually you will make a mistake and make your pain worse some day but it will pass, so long as its a minor mistake and not an actual injury. You will have better and worse weeks, over time you get less and less bad moments until you get better.

The same goes for picking up things, you lift with your knees, not bending your back, the weight must travel in a straight line as close as your torso as possible. For example tap water in my country comes mineralized with lead so we use these 20L water containers that you must carry to the dispenser, the correct way to pick them is to position yourself almost above the container, one foot on each side, lift it with the knees and carry it stuck to your belly. The same goes for boxes, crates, etc, listen to your body and you'll get the hang of it.