re: Bench press advice

The body is a chain. The legs start the lift, driving the arse into the bench. This force follows the arc of your back into your shoulders (rear delts, scapula) which then fires through your chest and into the lift. When you're getting into the lift, build the chain in the opposite direction. Pinch your shoulder blades back, activating the arch. Scooch your butt up to support maintaining the arch (this isn't some powerlifting super arch, just get that flat back off the bench). Finally, to get to your question about getting legs involved, lock it all into position by bringing your feet under you (act like you're trying to get them under your butt) and planting them. Knees should be bent to 90* minimum.

I see people benching all the time with their legs relaxed in front, just resting on the ground without much flexion. That's a guarantee that you aren't getting legs involved.

Getting into bench press position should be uncomfortable.