Today I probably won't have time to do a full on tutorial so I thought I'd share some information with you guys instead.
I recently bought some books from the super famous makeup artist Rae Morris and there's particularly one tip in there that is worth its weight in gold (the question is how much a tip weigh but you get my point ;) ).
Once you've applied powder to your face you can't do any touchups with cream or liquid products (such as foundation) later. So, if you know you're going to need to reapply foundation during the day, skip the powder!
I actually tried it and found this to be a crucial truth. Adding more liquid or cream product to an already powdered face will give an unclean and cakey appearance which you don't want unless you're going for the "my-face-is-falling-off"-look.
As for the Rae Morris books I will give you a review of them as soon as I've finished reading them both!