You just mix what you have in your storage room. It does matter the order! For example if you mix a stone with a candy, you get nothing but if you mix a candy with a stone such as a ruby, you will get a spicy red tonic. I have been getting random things even heart background for example. Just keep mixing whatever you got to see if it work and always try it both ways. 

OHHHH! Ok, I thought I was doing something wrong! Thank you!