Step 1: Press 7 on your keyboard
Step 2: Staying in X Z View, Create a wall with left click (Zoom out if you need too) (2 blocks tall)
Step 3: Press Ctrl -> Tab Twice (Gonna simply refer to pressing Ctrl -> Tab By saying it like this "Switch to top down view" Or "Switch to Front View")
Step 4: Press 4 then resize it till it is one block wide
Step 5: Press space and place the second wall at the other side of the floor
Step 6: Press space and click the Z on the bar at the top, then just line it up with the other edge of the floor
Step 7: Do the same as Step 5
Step 8: Shift Click the Floor (Select) then press space, Change to Front view then Move it up.