Hi. I actually edit my modpacks all the times and the things that you need to make sure of are that the mods you're adding are for the same version of minecraft and are istalled using forge. In general install the mod the same way you would for normal minecraft except use modpack.jar instead of minecraft.jar. Also, I've found exceptions where you can just put the mod in your mods folder regardless of how it should be installed. If that doesn't work than its probably a compatibility issue.
P.S. I'm no expert
