I have watched no end of videos by "experts" telling you how to do it - I watched one yesterday where he said "in one shot you will have perfect exposure" then went on to say all you had to do was switch over to spot metering on your camera, point it at what you wanted to photograph and hay presto you have perfect exposure, all though he did have to say that you will need to adjust the exposure for your subject as all subjects were different and none of them are 18% grey.
So I am going to tell you how to get a perfect exposure. Point your camera at an 18% grey card and take a picture of that grey card and yes you have a perfect exposure of a grey card.
Well no you have a good exposure but no perfect as your cameras reflective light meter, built in, does not work on trying to average out the area you metered to 18% it is more likely to be between 12 and 14 percent grey. My camera is calibrated to approximately 13.7%.
So you are screwed to start with.
My solution is always to use a separate hand held meter and use an Incident reading of the light falling on my subject. The reason for this is that I know I will always get a consistent exposure and one that I can work with.
So how do you get a "perfect exposure". You can't. You can get consistently good usable exposures using an incident light reading and knowing your cameras dynamic range, so at least in RAW you can work with that image to get an image that you are happy with.
Don't use the image on the back of your camera to do anything other than to give you an idea of the exposure and composition, as the image is a JPEG version with adjustments which your camera inflicts on you. The other thing that's gets me ranting is when "professionals" tell you to ignore the image and use the histogram to get the exposure right. What they either don't know or don't tell you is that the histogram is based on the JPEG with all those in-built adjustments, so it is not a true representation of the RAW image.
If you want to know how to get a good usable exposure first you need to find out the dynamic range of your camera, you will need to do the following. REMEMBER YOU MUST BE SHOOTING IN RAW MODE.
Find a subject wearing white and place it in an area which has good even light which will not change for a few minutes. Set you camera on a tripod and focus on your subject. Now take a spot reading off an area of white which is evenly lit. This is you middle reading, then you over expose your subject a stop at a time taking a picture at each stop. The go back to your middle reading and then at one stop intervals take a picture under exposing the subject.
My subject is Zena the warrior princess but don't worry she's armless.
I can recover detail on 9 of the images from my fist exposure, 4 over exposed and 5 under exposed. This is important because it tells me that there is less detail captured in the highlights than the shadows so when I take a picture and I want to get the best out of a scene, then all I need to do is meter off whatever is in the scene that is white and I want to retain detail in then meter off whatever is dark in the scene that I want detail in and see if those two readings are no more than 8 stops different, then set you settings to the middle of the readings. Not perfect exposure but usable exposure. So my middle setting was 1/20th sec at f4 ISO 64 the over exposed setting that I still could recover highlight detail was 1 sec at f4 ISO 64 and the underexposed reading was 1/640 sec at f4 ISO 64.
I bet you are now even more confused than before, the more you know the worse it gets, my advice for getting the best out of your camera is practice, get to know what it is capable of and get your self a hand held light meter and take incident light readings and you will never be far off a very good exposure and occasionally it will be perfect.. Good luck.