When learning how to slackline, it is important to first learn how to balance on the slackline and not how to walk on the slackline. In the first step, train to stand on one leg, keeping your back strait, your arms in the air and breathing deeply. Once that you are able to do it on both legs, start making your first steps, engaging the movement with your hips and not your shoulders.
At the beginning, your leg will be shacking a lot, it is normal. Because of the instability of the surface below your foot, your muscle get over tense. Breathing deeply and trying many times in a role will stop your leg from shacking. Just like learning how to ride a bike, it will take some times at the beginning but will quickly turn into a fun and challenging activity ! Everybody can balance on a slackline!