Here we can bring in the clothing. Remember to use Masks so you can make any adjustments later. Mask and place them on your model. Keep in mind things like fur around the collar and ears hanging down over or behind the clothing. Adding little pieces of depth can help add to the realism of the final product.
From here we can start adding shadows, these are really what will make this image pop and give it a believable feel. First let's make a hard shadow under the ears and collar as if the main light was coming from the top right. If you place your cursor over the mask for the dog and hold Ctrl or Cmd when you click on it it will select the mask. Bring in a new layer under the animal but above the clothing called "Hard Shadow". With the Layer "Hard Shadow" active and the Subjects mask as the selection, go to Selection > Transform Selection and move it down and to the left by just a little bit. Hit enter and use a nice fluffy brush at about 10% opacity to draw in a little shadow. No need to get too harsh here just enough to define the edge. Then deselect that (Select > Deselect) and bring in a new layer above "Hard Shadow" and call it "Soft Shadow". Here we'll use abut the same brush to draw in a fluffier shadow following the same line as the last one but without the hard edge. Let the Hard Shadow define the edge and the soft shadow gives some depth.
This step depends entirely on if you chose to add a hat or any other clothing to the face - if you haven't then feel free to move on. Go ahead and create a new layer on top of your subjects layer and call it Hat Shadow (or whatever article of clothing you're using) and just brush in a nice soft shadow under the brim of the hat. Don't be afraid to make it pretty dark right under it but work in 10% increments (using a black brush with 10% opacity).
Again this step is optional but I thought it could be useful as it helps keep things clean and organized and also helps with computer performance. Group all the layers together that are involved with your main subject by selecting them all while holding shift and pressing Ctrl+G on PC or Cmd+G on Mac. This will group them together. By right-clicking on this group you can convert it to a smart object. Making it work as one layer, however, you can double click on the thumbnail and edit it in its own window. A nice little feature that can be very helpful in certain situations.
Moving on to the overall lighting, we'll start with the backlight. Above the Background layer create a new layer (Ctrl+Shft+N on PC and CMD+Shft+N on Mac) and name it Backlight. Brush in white with a nice big soft brush a soft light behind the right side of your model, similar to below. This will help extenuate the highlights later.
Before going forward let's sharpen up our image to give it a real studio feel. This can really be done whenever but I chose to do it here. With the smart object layer selected go to Filters>Sharpen>Smart Sharpen, my personal settings can be seen below but your own image may require a different touch.
Next, bring in a new layer on top of the subject layer and name it "Highlight". Right-click on this layer and make it a clipping mask to the subject layer. This will make sure it only applies to the subject layer and not the Background. Brush in, with a big fluffy brush at about 20% opacity a little highlight on the right side. Set the layer's blending mode to overlay. Be careful not to overdo it, this will cause pixels to distort and end up looking bad later. A soft touch is encouraged here.