You can easily build new Walls by tapping on the Shop (shopping cart) then select the Decos tabs. Here, select the button "Object Type" and then "Walls", this will open all the different wall types available to use.
Once you have selected the Wall and it appears in your Restaurant, simply move the wall to your desired place and select the green tick.
You can redesign or erase existing Walls by tapping on any area of the Wall that is already in place until the tooltip opens at the bottom of your screen. There, you can select any new design option or select the Wall displaying the "X" to erase a wall.