...So I still need to create an adjustment layer, eg a saturation one...
Yeah, a Hue Saturation layer will de-saturate the image if thats what you want to do.
...then do I select the "Use previous layer to create clipping mask option"...
You can if you want to although with only the one layer it isn't really necessary.
The 'Clipping Mask' means the adjustment layer affects ONLY the layer BELOW it....but having only one layer below it, it makes no odds in this case.
...Then what other steps do I have to do, sorry to be so nieve...
No worries.
To be honest you are better off adding an adjustment layer by clicking the 'Create New Fill or Adjustment Layer' icon at the bottom of the layers panel...its the 4th icon from the left and looks like a half black, half white circle. (Right above the time in your screenshot!)
Using this will skip the 'Clipping mask' option dialog, (Which you don't need anyway), and just add the adjustment layer directly above the currently selected layer....again in this case there is only one layer so it should be added above that. (You should really make a copy of the original layer but the method here is non-destructive anyway so the choice is yours on that one)
You should see the adjustment dialog automatically open in which you can adjust the 'Hue Saturation', (if that is the adjustment).
Once adjusted click OK to accept the changes.
Note the empty 'Layer Mask' next to the adjustment icon....the arrow in my image points to it.
The adjustment layer MUST be selected BEFORE painting on the mask, the mask will automatically be selected for painting on.
Now, with the 'Brush' tool, and a greyscale colour, paint on the mask. (Your image).
With BLACK you will HIDE the adjustment....effectively turning it back into colour.
With WHITE you will SHOW the adjustment....effectively turning it de-saturated, (Or whatever adjustment you do)
You now just need to paint in BLACK everything you DO NOT want adjusted. You can easily do this with any of the regular tools, paint brush, pencil, fill, make a 'selection' if you need to and fill it with black.....whatever method you choose.
All you have to remember is BLACK will HIDE the adjustment, WHITE will SHOW the adjustment...Check my image, you can clearly see that the black part of the MASK removes the adjustment in that area. (You can paint over the black in white to apply the adjustment again)
AND DON'T FORGET TO HAVE THE ADJUSTMENT LAYER SELECTED BEFORE YOU PAINT ON IT !!!
(You can also double click the adjustment icon to open the settings dialog again if you wish)
Regards.
MrToM.