You have to be REALLY close to the restaurant to be able to join the walk up list. There really isn't any way to plan ahead for this way of dining. It is all based on how busy they are that day and whether they are even doing the walk up list that day. VERY often, it is not an option. It's quite unpredictable. I would consider this to be a spontaneous dining thing.
The best way to do this is, if you are hungry, or getting there, go on the app and select "check dining availability", enter your party number, select that day, and click on "Now". They will show you a list of restaurants that offer Walk Up wait list and show you their status (whether the list is open or not, and if open, how long the wait is). From there, if you find one that is open and nearby, you can join it. If not, keep trying. The absolute best time to join is RIGHT at the start of their service hours. You can practice from home right now to see how this works. They don't let you actually join unless you are within about 50 yards.