1. Open the picture of design you that want to modify
2. Save as new file (so that you still have a backup of the real design)
3. Create new layer (Ctrl + Shift + N)
4. Now you will work on the new layer. MAKE SURE the new layer is highlighted. (pict 4)
5. Use Brush Tool and set the opacity & flow under 100 (you can experiment on this). (pict 5) I used 85% & 75%... The lower you set, the more water color effect you got :p
6. Draw the modification. It's okay if messy. (pict 6)
7. To clean up the mess:
7.1. Set the opacity of the layer lower (50% I suggest), so you can see the base picture. (pict 7a)
a. Use eraser with opacity 100% & flow 100%. Erase until neat. Tips: you can click one dot then click Shift + next point... it will erase based on line (ah I can't explain... just try it hehe)
OR...
b. Use Polygonal Lasso Tool to select which part of the paint to be removed. (pict 7b) Tips: select small parts first then click Shift to add other parts. Click Alt to remove selection from some parts.8. Now you have neat modification
9. Use some tools like Burn to darken your painting or Dodge to lighten it (I used it to create shiny look).
10. Just keep on going for other parts of modification ;)
PS: sorry for not visiting your blogs... I'll try to catch up this weekend..
** source of picture (base model): Elle.com, Requiem collection 2009