The sheer amount of dust that can get inside a computer can be surprising.
The first step is to make sure the computer is shut down properly and not just in sleep mode.
Use the Start Menu, Power button, Shutdown option and wait for the screen to go black and all lights on the main unit to go off.
Turn off the power cable at the wall socket.
You will need to make sure you know where all the plugs go back in before you unplug anything. I suggest taking a photo with a mobile phone camera so you know how many cables there are and which cable goes where.
Unplug the main unit and move it to somewhere you can position it with the opening side upwards.
The way of opening the case varies by manufacturer. Some require a screwdriver and the removal of up to 3 screws. Others have a lever-operated catch mechanism.
Take off the panel and put it to one side.
It is important not to get the inside of the computer wet. Anti-Bac wipes are generally ok, but you should definitely not use wet cloths or similar. Air dusters are used to blow the dust out of hard-to-reach areas such as in between heatsink fins and around fan blades.
When cleaning the fan blades, try to move them slowly and hold them in position whilst cleaning. If they spin in the jet of the air duster, this can create a small electrical charge, which you don’t particularly want to occur.
Once you are happy that the fans and heatsinks are no longer clogged up with dust, the panel can be reattached, the cables plugged back in, and the computer powered back on.