First we need a map. (of course)

Next we need to create a .bns file with name of our map
Just create a .txt file. then open file>Save as...
and then type something like this
in that file you should type something like this.

And after this things you should see your map in bonus maps


And yes, you can actually put some console commands to the "map" quota like "funnybox; sv_cheats 1; god"

But you probably wondering: how can i add image?
Well, if you want to know, time to go to the next step

First, we should take a screenshot of our map

(sorry for small size of picture)
Next, we should name our screenshot as... you guessed it, like the map file

Open any program where you can edit images
I use the paint net
your image should be 180x100
Save as .tga file
looks like this


Time to look at it in game
NICE

Now its time for pack of maps

First, we need to create folder in maps\
I gave my funny folder funny name:
Next we need to create .bns inside this folder. But this time we need to create folderinfo.bns

in this file we need to type something like this
Lock quota means that it will locked or not. But i think you will use the 0, instead of 1.
And you can also create an icon for it.
name it foldericon.tga
(the same resolution as for the maps)

And for the maps, just throw .bsp, tga and .bns in your map pack.
But, as for .bns you should change its image quota to your map pack folder path
like this

Here's the result:

Thanks for reading this guide. Hope it was helpfull
Good luck in making awesome maps!

Also you can check what you can do with this bonus maps feature in Valve Developer Community

(and yeah, don't forget to like this guide. please.)