just try all the settings, some of them will look better than others, but you have to lower the opacity a little and it will then show through some of the light and shade of the car panels, your window ones look ok, with the skirts/bumpers, some of them just wont work very well, it is always better to have more panel on the skirt/bumper than is needed, then cut it out to the lines that the original car has, then use the eraser tool to blend out the panel edge, then you have to hue/sat and bright/contrast to get it to look as close a possible, then in some cases you will have to burn/dodge the added panels to add shading in the right areas (with the rear bumper you needed to burn the top of the bumper for it to match the darkness of the original car, keep the burn setting down to 4 or 5 with low flow, you will have to make a lot of passes but slower is better so you dont over do it, also you could erase the edge with around 10% with around 15%flow which will bring some of the original shading/colour through)
keep trying it all gets easier, i have only been casual photoshoping for around a year now, keep looking up some of the tutorials they always help
this one at carchops is quite good -
car chopsand a big list here -
photoshop tutorialsgood luck