Okay, I'm going to give you the Modpack creation 101 tutorial.
1. Determine if it will be public or private. Public wil be released and private is for yourself (modding vanilla minecraft on your own counts as making a private modpack without launcher). Private includes friends.
3. If it's a public modpack or a private shared modpack (distributed to your friends), ask permissions to each modders before putting up the modpack together. You want to cover your back if any legal stuff risk hapenning.
5a. To resolve IDs, you create a notepad where you copy paste the block/item ID of each contents added by the mods. You then edit them to prevent any ID errors after that. Save that list, it's going to be a lifeline if you ever want to update or change the modpack. Copy the new IDs into the mod config files (been a while since I last checked where they were, so you will need to figure where they are by yourself).