Where should tag proposals be made?
This is meta, anything about the site can be posted here if you are unsure or just curious, so you can post those here in the future and the community will respond.
Once you have the privilege to create tags, you can do so if you think your post is unique enough to merit a new tag.
Don't abuse this though, we keep strict tabs on the tags already in the system and as they are added. Often times you might find that your new tag can be generalized under a broader tag or the other way around. That being said, you can also just leave a comment under your post suggesting that the tag be added and someone with the privilege will add it if it's necessary.
Additionally, you are not obligated to do so, but if you create a new tag, again, feel free to create the tag wiki for it.
If you do not have the reputation to create a new tag, you can just leave a comment on your question asking someone with enough reputation to add the tag to your question for you.
There's even a badge for creating a tag used by 50 questions.