1. First Download your extra item. Let's say we want to add this plugin.
2. Then, we will have to compile the .sma file.
3. Add the compiled file .amxx file to amxmodx/plugins folder.
4. Next, just go to amxmodx/config/plugins-zplague.ini or plugins-zp50_ammopacks.ini and make a new line and add the plugin name. Example: SpearGun.amxx
5. Now we'll have to add the sprites, models and sounds. First off we'll add the sprites, go to cstrike/sprites folder and add the sprites that came with the extra item. Next, go to cstrike/models folder and add the models that came with the extra item. Lastly, go to cstrike/sounds folder and add the sounds that came with the extra item.
6. Now, you will have to restart the server or the map.
If you have any problems with the extra item, then you'll have to contact the author of the plugin!