Make sure the torn area is as smooth as possible with no ragged pieces that will lift or move around when you apply the joint compound.When the compound dries and you sand the area you'll never know the difference.Actually,I use a damp sponge instead of sandpaper...much neater!

I feel for you. I had the same problem because I dug too deeply with the putty knife when removing the wallpaper. Use regular joint compound. Hold the taping knife so that it is almost parallel to the wall so as to get the smoothest finishing coat without lifting up any of the loose drywall paper. Doing so smoothes the compound out better and smoothes any paper fragments out better into the compound. Otherwise, the drywall paper fragments will just appear through the joint compound, as well.

For future use, make sure you hold the putty/drywall knife also almost parallel to the wall when removing the wallpaper, and never force it. If it does not come off easily, then soak it some more and let the water do the work, not the putty/drywall knife.