when I divided the lower limit by the upper limit for ex., 254-3= 251. Then I divide 251 by the number of classes which is 251/7=35.85, that I then rounded off to 36.
what if the class interval is 36?
The class interval tells you the distance between each successive class' bounds. For example, if the lower bound of your first class is 3, the lower bound of your next higher class is 3+36, or 39. The 3rd class' lower bound is 36 more again, or 75. So your bounds would end up as:
The upper bounds for each class were calculated as follows: if 254 is the upper bound of your highest class, then the preceding class' upper bounds are each, simply, 36 less than the one above it.
Now you can use these bounds to complete your histogram, assuming you have a data set you are summarizing.