I am hoping you have prepared yourself for this tutorial by reading the first page, if not go back and read it!
Ook scroll through your girl's code and look for #2 #1;rosy cheeks:
The 46's circled in blue are the colour, the 35's circled in green are the size. Use Carolyn's Colour Chart to pick a different colour.
If you want to make a birth mark, simply make one of the 35's into a .0 and change the other into about 10. Then change the 46 after the 10 into a darker brown.
Let's have a look at what you have done by picking a code from Small & Pampered (make sure it is a baby that matches to the type you are hexing), save the Hex Workshop, say no that you do not want to make a Back Up and then open babyz and enter the code. If it doesn't work first time, try a different code. Please make sure to pick a code with rosies!