Ok, I did not find many tutorials that addressed this issue, at least any that I understand well, so I wrote this to try to help you ...
I wrote this tutorial based on some other tutorials and based on my experience.
If I have wrote something wrong, please correct me!
And sorry about my english...
->Set the game resolution to 1024x768;
->In the menu, open the console then type "sv_cheats 1","dev_overview 2","map yourmapname";
->Then the game will display a overview of your map, adjust the map to fit in the center of the screen, using D-pad/WASD and mouse to adjust zoom;
->Then take a screenshot with F5; (Don't use Print Screen SysRq)
->Type in the console "dev_overview 1", copy the information. (The console still update more times but the information will be the same; (If you don't move the map while do this)
->Type this: ***(Obs: Template for a real one)
// Overview description file for yourmapname.bsp
// (Overview: Zoom 2.42, Map Origin (452.00, 128.00, 336.00), Z Min 897.00, Z Max -225.00, Rotated 1) //Here says "Rotated 1"
ORIGIN 452.00 128.00 -225.00 //Take the "Z Max -225" value and put here!
ROTATED 1 //Then here will be 1
HEIGHT -225.00 //Take the "Z Max -225" value and put here!
->Save it and rename to the same name of your map; (only the name, no extention! (like.bsp))
->Grab the screenshot in the "cstrike" or "czero" folder;
->Open with a Image editor of your choice;
->Fill the background in green with Black (000,000,000).
The background is green (absolute green 000,255,000) in the "authentic" CS map overviews, but is better because you don't need to re-fill the green area of the image with 255 green (000,255,000) again after the convertion to 8 bit.
->Save the image to 8bit bitmap (.bmp) (256 colors) OR Targa (.tga) type 10 or 2. (Targa type 10 is truecolor);
->Rename the image to your map name (without the extention ".bsp");
->Grab your "yourmapname.txt" and "yourmapname.bmp" and put in the "cstrike/overviews" folder;
->Run the game! :D :D :D
NOTE: The image size will must be 1024x768, .bmp (8 bit) (256 colors), or .tga type 10 or 2! This is very important!
* "r_fullbright 1" to NOT display the map shadows (lightmapping).
* "hud_draw 0" to NOT display the HUD.
* The console opens with " or ~
* Don't use MS-PAINt :D
Please correct me if i missed any part.