Foot positioning, before you even attempt to pick the bar up, you need to ensure your feet are positioned correctly. Shoulder width apart, your feet should sit under the bar so it is in line with the balls of your feet.
Start position, now that your feet are set up and ready to go you will have to lower yourself down to the starting position of the rep. Engage your core and keeping your chest high (focus your eyes on a point straight in front of you will help with keeping your chest up) lower your hips until you can reach the bar with your hands. (If you find this position uncomfortable then check my regression exercises to help you work up to this point)
Once you are in the starting position, keeping the upper arm relaxed and shoulders retracted, drive your feet into the ground forcing yourself to stand up. At the top of the movement your core should still be engaged, and your shoulder should be relaxed. (no shrugging at the top of the movement)
Performing the same ques as we mentioned, lower yourself back down to the ground into your starting point, ready for the next rep.