Firstly, make sure that you have enabled the
DEVELOPER CONSOLE in the properties.
Now, change your resolution to 1280 x 1024
or else it WON'T line up, even if you do everything
else right!
Load up your map, make sure there are no bots
or things to distract you, then type a series
of commands into the console.
I recommend typing all of them, but I'll just
explain what they all do:
The top three are compulsory and need to be
entered in this order.
Cheats need to be enabled to be able to go
into noclip, noclip lets you move and position
your radar and cl_leveloverview sets the camera
above the level.
More on this in a second.
I recommend cl_leveloverviewmarker 1024 as
well, as this makes a red square that represents
the overview texture and helps you to line
your view for it for it to fit inside.
These two are pretty much compulsory as they
remove your HUD and weapon model.
r_skybox 0 stops the skybox from being rendered,
since this should not be part of the level's
overview.
You need to type these two commands in on
separate lines and in this order- the first
lets you change the fog and the second changes
it.
This will make the level overview clearer
if you have fog in your map.
r_drawstaticprops 0 removes things such as
trees and other props from your level, should
you not want them showing in the overview.
mat_postprocess_enable 0 removes the bloom
effect.
cl_leveloverview 8 and cl_overviewmarker 1024
will show this screen.
Your aim is to fit the whole map within the
red box.
This can be fiddly and the result is dependant
on where your character is within noclip.
Thankfully a bit of messing about with WASD
and the height and direction in which you
were standing before typing the commands tends
to deliver a reasonable result.
Simply type cl_leveloverview 0 to go back
to your character's view, should you need
to to try another position and direction.
If the map is too small within the box, type
cl_leveloverview again but with a smaller
value.
Do the opposite if you can't fit it all in
to the 1024-sized square.
Once you have something that looks okay, like
this, it's time for the next stage.
We need to make two files- one is the picture
of the overview, and the other is a text file
telling the game how to line it up.
There are a billion ways of getting these
into files, for the picture I just take a
screenshot of this view and crop it in paint
to 1024x1024 to remove the red box outline.
With the coordinates found in the console,
I copy and paste them into a text file then
make it resemble this, only with my map's
filename and coordinates.
You can find a template for this within the
description of this video.
The only rule is that once you've got an overview
position you like, you don't move it, as this
will also affect the coordinates in the console
that are needed to line them up.
Once you have both the picture file, and the
coordinates for it saved, you can close the
game.
Name the image file to your map's name, plus
a _radar at the end.
Name the txt file to be your map's name.
Place both of these into the Steam\steamapps\common\Counter-Strike
Global Offensive\csgo\resource\overviews directory.
Sadly, Valve decided to make life hard for
all of us and demand that the map's picture
needs to be of DDS format.
How you do this is up to you- I used Gimp
2 and downloaded the DDS plugin for it, placing
it in this folder: GIMP-2.0\lib\gimp\2.0\plug-ins
.
You then load up the overview picture file
in Gimp, go up to FILE, EXPORT, and save it
as a DDS format.
Loading it up in-game should now 
display the radar both on the loading screen,
and when playing the level itself.
Still having problems with it not appearing
or lining up properly?
Read the description.
Have fun and... aren't those shadows pretty.
http://registry.gimp.org/files/gimp-dds-win32-2.0.2.zip
