You can divide 36 by 12.  12 goes into 36 3 times.  Take each ingredient and add 3 times that ingredient to your mixture.  Imagine the circles below are cookies:

       O    O    O    O    O    O    O    O    O    O    O    O                  

Add 2 more cookies to each cookie. 

                      O O O O O O O O O O O O

                                        +

                      O O O O O O O O O O O O

                      O O O O O O O O O O O O

How many do you have?

If we assume that you already have a list of ingredients to make 12 cookies, and that each of the 36 people will eat one cookie each, we can use use simple division and multiplication to figure out how much of each ingredient we need to make 36 cookies.  

First, let's start with the division question:

   How many times does 12 go into 36?   or    What is 36 divided by 12?  
   The answer: 3
   This means we need 3 times the amount of ingredients to make 36 cookies as we did to make 12.

Then comes the multiplication part:
   multiply the original quantity of each ingredient by 3.

For example, if the original 12-cookie recipe calls for 1 egg, we will need 3 eggs to make 36 cookies (1 x 3 = 3)