Hi @gokhanamal,
I added some links to my reply to Implement Semantic Highlighting for SourceKit-LSP - #2 by blangmuir that may be helpful when you look into this.
Are there any things to do before the proposal deadline?
I don't think there are any specific requirements before the proposal (CC @akyrtzi who will know best), but as you look into this area it's good to stay in touch here on the forums to make sure you're on the right track, or to answer any questions that come up.