
Set up your gimp space with 2 images. One with a naked bimbo and one with the items you want to combine.

Starting with the top part, use the lasso tool to draw around the top. Right click and copy once you've done this. Then go to your naked bimbo, right click > edit > Paste as > New layer. Its really important you paste this as a new layer, you'll see why later

Now with the bottom, again use the lasso tool but also remember to include part of the dress a little above the area of the top. Again, right click and copy, then right click > edit > Paste as> New layer.

Now as you can see, the purple skirt overlaps the belt of the top. Switch the purple skirt layer with the white top (just drag the purple layer down on place)
And this is the final result