do you add it instead of something else, water or oil?

I used it in addition to the regular ingredients in a box mix. I've never added it to a scratch cake though.

I believe this is the recipe I used to use: http://cakecentral.com/cake_recipe-1977-Cake-Mix-extender.html

i'm going to make a strawberry cake and i want to try the s/c to it but i just don't know how?