Your thread is filled with awful advice from non-boxers.
When you throw the jab, your weight goes from the back foot to the front foot. You know how when you run, you are gripping the ground and bringing your weight forward with every step? It's the same concept. You should be balanced on your back foot then run the weight forward with your front foot, then immediately back to the back foot after you land the punch. It's how boxers punch from just outside of range without leaving themselves liable to counter.
Some boxers fight purely peek a book style with their hands held high but it is far more natural for bipedal human beings to stand facing sideways. "Slipping right" is actually bending down; "slipping left" is actually pulling yourself back. So in this sideways stance your shoulder blocks the left side of your face, but you rotate the whole torso slightly to block punches, you don't just try to move your shoulder joint. Floyd Mayweather has a perfect natural stance.
When you throw the straight right hand, you are again rotating the whole torso, pivoting on the back foot like you are trying to stomp out a cigarette. The actual arm punch motion is the last part of the whole punch. Holding the right hand high near your chin, the hand shouldn't move until you are ready to snap that final straight line right through the bag. If you move the hand off the line you throw a slap punch and lose speed and power. Guillermo Rigondeaux throws perfect straight rights.
Throwing hooks you are trying to pivot your body weight into the bag. Pivot the right foot for right hooks and the left foot for left hooks. It's not important whether your knuckles face down or sideways, this is just to gauge distance of the hook while maintaining balance, but don't worry about that, when you reach that level it will feel natural. Gennady Golovkin throws perfect hooks.
When pivoting, you should be able to really stomp that weight onto your toes and maintain balance. Balance is everything as anyone will tell you.
Don't go apeshit when you throw combos. A lot of people giving advice to beginner boxers seem to really want to defang them, but it's an aggressive violent sport. You can lay into a bag as hard as you want but hold the balance firm when you combine your punches. Doing this swiftly and with power is tremendous exercise for your whole body and you will become extremely coordinated and strong if you stick with it.
Don't search "boxing tutorials" "begginer boxing tips" all fucking garbage. Joke dudes in front of cameras. Watch professional fighters do their thing. Particularly the names I gave you.