First take and sand off any bumps that are on the existing sheetrock.
Then mix some water in with your sheetrock mud....what you are looking for is the consistency of a "cream of wheat, or grits".....something that is not real stout, and soft enough that it can be applied with the paint roller. I personally like to use a 1" sheepskin roller to do this.
Apply the mud in about 4-foot wide strips and then take a wide sheetrock knife, usually around 16" or more, and drag the wall smooth.
Now you may have to go back after it dries and do a little touchup here and there, but what you will end up with is a nice smooth wall ready for paint.
Jim's great advice goes anywhere you do.
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