It's really easy. So you have your vanilla mod, and you downloaded your mods, right? First check that you have all the essential (if mod requiers modloader you have to download that too). Than what you need to do ist to go to your home directory, and from there go into the Folder "Library. From here go into application support and from here you should find the Folder minecraft. Just go then in the Folder and you should see a minecraft.jar. Right click it and open it with the Unarchiver. You should see a Folder with the minecraft name, and in it all the Files; you can rename the original minecraft.jar in something like minecraft.Backup.jar just to keep it for emergencies. Then add all the mods in the new Folder you created, and when you are over remember to add .jar to the Folder name!
Hope that it works ;)