Thanks!
I took two screenshots: one wearing the complete outfit and one without the actual dress, so I could see the character's body shape, and also the gloves and grey background.
In Photoshop I had three main layers - top to bottom:
- a cutout of the upper skirt and ribbons
- screenshot with the dress
- screenshot without the dress
And to put them together:
1. Put the two screenshots into Photoshop and duplicated the dress screenshot. Turned 'dress' and 'body' layers to hidden.
2. Skirt layer: used the Polygon Lasso tool (in small segments) to cut out the skirt, or rather to cut out and delete chunks of the rest of the image around it. Was left with just the skirt and some grey background.
1. Turned on body layer and moved the skirt into position.
2. Turned on dress layer and erased the section of the original skirt that was still visible, revealing the gloves and empty background of the 'body' layer. (I had to be careful around the gloves because it turns out they didn't line up quite perfectly - either from basic character animation or I accidentally moved it a tiny bit between screenshots)
4. Skirt layer: erased some bits of grey background around the ribbons where it was now overlapping the skirt
5. Dress layer: cut out and copied the thin section of undecorated bodice, and duplicated it a few times to fill in the gap. Did left and right sides separately, and also had to rotate the pieces slightly to line them up.
Unfortunately I didn't save the actual Photoshop file, or I'd post screenshots of the layers.