When Installing models, go to your cstrike directory in which you have installed.
"_X:\Half-life\SIERRA\cstrike_" <-- Note: Assuming your drive where you installed cs is in your "L**ocal Disk C or etc.**"
Inside the cstrike directory, you can see multiple folders. Now copy the model you want to use and paste it in your cstrike directory and move it to the "**models**" folder.
Installing sounds is putting your new sounds in the "**sounds**" folder in your "**cstrike**" directory..
**If replacing a player model:**__
The same thing: Copy the player model you want to replace and paste it in your cstrike directory then move it inside the "**models**" folder then go to the "**player**" folder and replace any model you want to.
Folders inside the "**player**" folder: