What consistency should drywall mud be?
I found it took me a couple hundred hours of applying mud to get to the point where I knew exactly how I wanted it to look.  I learned just fine by starting with the pre-mixed straight out of the bucket and slapping it on the wall.  You'll quickly notice the difference in application ("spreadability") when it starts to dry in your mud pan a little bit.  Then you add water - too much the first time of course - and the mud will fall off your knife before you can get it to the wall.  

Just keep in mind, it might take more than two or three coats, but if you can spread it on the wall you can't really screw up the consistency.  If it's too dry, it will crumble as you press it with the knife, if it's too wet it will fall off your knife (and the wall).