There are many ways of mixing or not mixing. Hmm how should I explain? let it come naturally,let it be loose at first.Give it multiple tries from differentdirections.
YOu can approach it like the impressionists, by simply not mixing them, just putting dabs onto eachother and letting them mixing in the eye , heres a Monet:
http://lh6.ggpht.com/_goEzH8QTahQ/SJtaynH4XxI/AAAAAAAAA2c/ZGv6fNoNL6s/Claude+Monet.jpg
or like a classicist, mixing the colours to total perfection on the canvas:
http://getdagoss.com/blog/wp-content/uploads/2009/07/death_of_marat_by_jacques_louis_david.jpg
Or search on Hyperrealism which is even more detailed, or Delacroix, who was a romanticist, who has a very nice way of blending....
There are many different ways, just experiment. If you're looking for a smooth touch, which I would guess is what you goal is, then it's bets toexperiment, by mixing the paint both on the paper and blending them there or mixing them on a palette before actually adding it to the picture. i suppose bestis to use both that should result in a neat touch. Also another thing is, try other mediums as well, oil paint mixes beautifully since it doesnt dry and itseasy to achieve tones. Or water paint mixes again, totally diferrently. Also ask your art teacher what he/she thinks, since in life a painting is 110 percentdifferent then on a picture unfortunately, but regardless kepp posting.