First of all you need two calculations with CHGCARs.

  from siman.chg.chg_func import cal_chg_diff
  cl1 = db['Your_first_calcualtion.with_CHGCAR.1']
  cl2 = db['Your_second_calcualtion.with_CHGCAR.1']
  cal_chg_diff(cl1, cl2)

The generated CHGCAR with charge difference can be found in the 'Your_first_calcualtion.with_CHGCAR' directory with the name like the following one 'CHGCAR_Your_first_calcualtion-Your_second_calcualtion'

3D view:
Open the generated CHGCAR in VESTA.
You can change isosurface view in Style > Properties > Isosurface.
Here you can change the isosurface level.
The higher it's setted the less amount of the isosurface you'll see.

2D view:
Open the generated CHGCAR in VESTA.
Go to Utilities > 2D Data Display.
You can change the amount of visible isosurface by tunning the saturation level.
For the start set the saturation level as:
Max … (100 %)
Min … (0 %)

Done! Enjoy the visualized charge difference density!