How exactly we can adjust the aparature in SLR camera?

How to adjust the shutter speed and aparture of Nikon D5100 for best result for night portrait?

Put it in aperture priority mode and turn the control wheel or put it in manual mode and press the AV button while turning control wheel. This is how it works on most cameras but admittedly I've never used that specific one.

1. Set shutter speed to 1/(1.5FL)
2. Set aperture to its max value (smallest number)
3. Set ISO to match the exposure value of model's face
4. Use flash for fill in lighting (light model's face). Use -1 flash compensation.

Since you have not mentioned using flash.
to be short, switch to AV mode or aperture priority.dial to its lowest number and set ISO to maximum.
the shutter speed with automatically adjust. And shot!

Because you sound as if you are just learning how to use the camera I'm going to suggest something different to everyone who has so far posted.

Put your camera into it's "P"rogram mode, and set the ISO to auto. Now when you turn the control wheel the camera will automatically select Aperture and matching shutter speeds. Turn the wheel till the aperture value is a low as it can go (low f/number = large aperture), and keep on turning it until the shutter speed is acceptable (around 1/30th should be okay) - having run out of aperture adjustment it should now be upping the ISO as the shutter speed increases. Give that a go, and please remember to reward your points based on the advice that works.