Linked Lists in C

Ok, i have a programming assignment due tomorrow, and I can not really grasp Linked lists very well. Well, i mean i understand what they are and know some of the syntax, but ive been staring at my computer for the last few hours dumbfounded as to how to add or subtract 2 seperate linked lists. Thats my assignment, i have to read 2 numbers from a file into seperate linked lists (in reverse order), read whether they should be added or subtracted, do the function and save new sum/difference in a new linked list and print it out. Apparently this is in case of large integers that can't be stored as just int. There was a example posted that added 2 linked lists of the same size, but it doesnt work on different sizes and doesnt work for subtraction.

I feel dumb when programming classes make me unsure how to do basic math with a computer.