It looks like you have enough joint compound on that to sand it down already... but I can't be sure from a picture. Yes, if you are going to apply more joint compound (I like to call it mud), you should probably sand that, or at least use a damp towel to knock down the really rough spots.
If you aren't familiar with the right consistency, buy some fresh mud and compare your consistency.
Also, a couple more tips: don't scrape your putty knife (trowel or whatever) off into your bucket of unused mud, just throw it away, because inevitably, you will put little chunks of stuff back into the fresh joint compound, making it difficult to apply smoothly. And you generally want to apply joint compound in an upward motion whenever possible... it just helps.