Allthough you should use the search button first, here we go:
Normally the crack you dled is compressed. It has an extension like .zip, .rar, .ace.
Now go and search (google....) for programs called winzip (for zip-files), winrar (for rar-files) or winace (guess for what kind of files!) and decompress the crack you dled with these tools.
Read a file called readme.txt xxx.nfo or xxx.diz included in the archive.
If it is a patch ==> backup your original exe
copy the patch-file in your game directory and execute it ==> if everthing goes fine your game will not ask for the cd anymore (or just click ok. if it asks for it)
If it is a fixed exe you still need a backup-cd to play the game. (Copy the whole contents of the original cd to hd and burn it back onto a cd-r, don't forget to label it as the original cd)
If it is a no-cd exe, simply copy the exe into your game directory. Be sure it has the same name like the original exe.
IMPORTANT!!!! Always backup your original exe before doing anything.
Hope this helps.
Be sure to check out:
http://gcw.abs-x.com/games/gcw_newbie.shtml