The Full Spectrum System Scanner is an integrated module included in all ships. It is activated after switching to Analysis Mode and conducting an initial scan of a star system using the Discovery Scanner to identify all astronomical objects and signal sources. The FSS Scanner is then used to map and obtain detailed data about each identified location .
All the bodies in a system emit a low or high energy , depending on how heavy this object or stellar body is the emission can be in low or high energy range . for example very high energy emissions could be from a very large , ringed gas giant while the moons that orbit around that gas giant have lower range emissions .
in the following picture you can see an overview on what i mean ...

when you enter a new system you might want to scan it to do that you need to enter the FSS Scanner ( you can find the specefic key in the key mapping section of game options and controls )

note that you MUST be in supercruise mode and in a low speed to enter FSS .

Once you have entered the FSS mode first thing you should do is to hold the mouse3 ( middle mouse button ) for the scanner to emit a energy pulse which will partially scan the system , drawing the orbital lines and it will indicate the index energy signatures in the system.

some systems are singular bodies meaning that there is only 1 planet orbiting the system's star in these systems the high energy pulse will complete the scan for that system, in other systems you might find it difficult to scan the whole system .

okay now that you have your index energy signatures in the system you must start scanning .
how to find a body?? very easy! if you look at the middle button part of your screen you will see a spectrum ranging from low ( left part ) to high ( right part ) .
now you must take your indicator to the places where there is a agitation in the line now just move your mouse around the highlighted parts of the space until you see a full or dashed circle . the wave forms you see around your reticle are a guide just follow them to find your emission once you located the circle , if it was full click on it and if it was dashed , carefully using your right and left or A & D keys move your indicator until the circle is full and then click on it now you either find the body or must zoom in on another level to find it ( zooming in again is pretty much the same - just do the steps as before . ) zoom in until you see the planet and it is scanned ! .

now if your planet was a heavy planet e.g. ringed gas giant , there is a high chance that the planet has moons orbiting it .

moons have lower energy emissions than the planet itself so slowly lower your indicator until you see those waveforms around your reticle again then do the same steps as before and boom you have scanned a planet and it's moons .
do the steps for all energy ranges until your system is 100% scanned .

if you navigate to the lowest part of the spectrum ( the far left part ) you can see a lot like A LOT of emission throughout the system . this emissions are not from a planet or moon these emissions usually belong to " degraded emissions " , " conflict zones " , etc. and they do not count as a body for the system .

if you are stuck scanning and you can't find a body just aim at a orbit and move your indicator until you see a highlighted area - then you can narrow your search .

most of the times the bodies are located on the orbital lines how ever there is a chance that a body doesn't show on those lines which you should search the space for highlighted areas that belong to the body and once you scan the body you can see the orbital line for it .

once your scan is complete you can sell the data at any station or starport . there is only 2 conditions :
1- the system must not be already scanned .
2- you must be in a 20LY or more radios from the system you scanned to be able to sell the data .
to sell the data , land on anything station or starport then navigate to starport/station services -> universal cartography -> sell page ( note that if you click on sell page , it will sell all data at once . ) .

it is easy ... remember those low emissions that were all around the place ?? now is the time to use them . let's say that you are on a salvage black box operation . and you have entered the target system , here must find it .
enter the FSS Scanner and do the initial scan of the system , then take your indicator to the far left and now you see a lot of wave forms around your reticle belonging to " degraded emissions " these emissions usually belong to a crash site where you can salvage stuff . all of them are colored yellow and their waveform is in white color BUT your target has blue waveform and it's icon unlike the other emissions is a blue sphere .
now that you have found your target press T to lock on it then exit the FSS Scanner and supercruise to the location .
there are some more emissions that exist - if you wanted me to explain just let me know in the comment section . :)