We'll do the opposite on the left side. A new layer named "LowersideShadow" with a clipping mask down to the subject's layer. Do roughly the same as above but with a black brush.
Repeat the last step but this time with a normal blending mode and focusing more on the upper left than the lower. It may seem tedious but layering here will help create nice realistic shadows.
Here you can do the other clothing items and accessories. I chose to focus on the hat and add a slight shadow to the left just to give it a little bit more shape.
Now that we have some of the basic shadows down we can use adjustment layers to really deepen the shadows and highlights. We'll start with the shadows. Go to Layer>New Adjustment Layer>Exposure. Again make it a clipping mask. Below you can see my settings to get a nice deep shadow (however again yours might be different). This will darken the whole image, to change this you can use the same asking technique as before on the adjustment mask by selecting the white mask thumbnail on the right and painting in black the areas where you don't want the adjustment. After painting this in you should have a nice smooth and dynamic shadow on the left similar to below.
Now we can do the opposite for the highlight on the right. My exposure settings are below. Bear in mind that upping the exposure can very quickly distort colors and pixels so I even lowered the opacity of mine to ease it in.
A minor step but a little detail that adds a bit to the overall portrait is the reflection of the lights in the eyes. I simply used the Elliptical Marquee tool (M) and draw in some white reflections on a separate layer. Again this is minor but helps bring the eyes out a bit which is very popular in-studio shoots.
From here we can get into some overall color editing. This part can give the image its overall mood so your direction may differ from mine and I encourage you to experiment and play with adjustment layers to get the effect you're looking for. I like to start with a Levels Adjustment (Layer>New Adjustment Layer>Levels).
Next a little black and white (Layer>New Adjustment Layer>Black and White) with a lowered opacity can give it a darker mood and a more "Classic" look.
However I am not going for that look, but the black and white layer placed before the adjustment colors will give an interesting look. So let's bump the vibrancy back up with a Vibrancy (Layer>New Adjustment Layer>Vibrance) layer to get some of the brighter colors to pop for our gradient maps.
Finally, I usually end with Gradient Maps (Layer>New Adjustment Layer>Gradient Map) to unify the overall image with a similar set of colors, the gradient presets work very well here and are a popular choice for myself but must be eased in as Overlay blending modes and lowered opacities.
And we're done! Congrats on you're image! Hopefully, you learned something new here today and had fun creating this humorous and whimsical image!