To construct a Decisions Table follow theses steps.
1: Draw boxes for the top and bottom left quadrants
2: List the conditions in the top left quadrant. When possible, phrase the conditions as questions that can be answered with a Y for yes and an N for no.
( If your doing it in Word or Google docs you can go to insert and on the far left click tables and make it to what ever size you want.)
Hope this helped, let me know if your having trouble!