I know you will be freaking out once its on but have a look at the
sargent and the darks and lights, totally black in the deepest folds and pure white at the other end. I used a mix of mahogany, sand, and elfic flesh inbetween. there are better examples but Im pretty pleased with this guy.
You can use the Iraqi sand (IS) as the middle tone that will work well. Others may have better ideas but what I would do is use the IS then mix up some of the Ashen Brown(AB) with 50/50 black and take it from there, if your brave you can take it further. Water down your paint 75% water it will take a 5-6 coats to get the darkest shadows but every coat you do make it a smaller and smaller area, so only the deepest folds get all the layers. Once thats done mix some IS and AB together and over lap your darks a bit and slowly lessen you AB to full IS then do the same with the whites on the tops of the fold instead of the recesses.
You will find the whites Harder one of the best tips that gave me instant improvement was the lighter you go the more water you use. wipe of the excess on a paper towl you dont want it pooling, you should be good to go!
Oh make sure to imagin where the light is hitting the mini as you need to make the sadows uniform.
Have fun!