Step 1 : Copy the background layer ( Dialogs -> Layers -> Duplicate Layer [duplicate layer is the button on the bottom with the two sheets of paper] )

Step 2 : Gaussian Blur top layer ( Filters -> Blur -> Gaussian Blur [I use 8 px ] )

Step 3 : Divide Layers ( Go to your Layers Dialog and under the Mode drop list select Divide )

Step 4 : Flatten Image ( Layer -> Merge Down )

Step 5 : Apply Cartoon filter ( Go to Filters -> Artistic -> Cartoon and play with the settings till you are happy with the preview, then hit OK )

One thing that makes cartoon drawings stand out from other drawings is the heavy use of black lines. Every wall, all the items in a room are given dark black outlines in a cartoon.