1. Download the custom map to your PC - in this example we are adding mp_ww_sand.iwd found here: https://www.customapscod.com/map.php?id=MTY4Ng==
2. Use FTP to move the CUSTOM_MAP_NAME.iwd file inside the folder named "main" of your server
   
   If you are unsure how to use FTP please follow this guide: https://pingperfect.com/index.php/knowledgebase/19/Accessing-your-files--File-manager--FTP.html
3. Click "Configuration Files"
4. Click "Text-Editor" next to server.cfg
5. Scroll to the bottom
6. Find the set sv_mapRotation line that corresponds to the game mode your server is using.
7. Add the custom map similarly to this example:
   
   Before:
   set sv_mapRotation "gametype sd map mp_breakout gametype sd map mp_brecourt gametype sd map mp_burgundy
   
   After:
   set sv_mapRotation "gametype sd map mp_ww_sand gametype sd map mp_breakout gametype sd map mp_brecourt gametype sd map mp_burgundy
   
   
8. Save the file in the top left and startup your server.