#1: Make bottom screen available with "XSCREEN 2" or "XSCREEN 3".
#2: Do "DISPLAY 1".
#3: Decide location to print text with "LOCATE". The range of X of bottom screen is narrower than that of top screen (0-39).
#4: Print text with "PRINT".
Actually, you can choose operation object screen (top or bottom) exclusively with DISPLAY command. If you write text on the bottom screen and you want to operate the top screen again, you should do "DISPLAY 0".
Try writing "DISPLAY 0" in line 152 (above "WHILE 1").