Once you have standing and balancing on the slackline figured out, you're ready to take a couple of steps. The first few will be really intimidating, but it's as simple as you would think it would be. Look ahead rather than looking down and put one foot in front of the other. Take it slow and don't rely on your momentum from the previous step to keep on moving. You should be able to easily stop and go back to standing still after every step.
The middle of the line is the most difficult place to walk due to the line having the most wobble and sway there. If you start at one end and make it past the middle, getting to the other end of the line isn't bad.