First you click on the button 'Idea' on top of the neighbourhood feed, you can then create a message. Explain clearly what you have come up with and how your neighbours can help you implement it. You can make further arrangements by placing comments under the original message. If you only need to agree with a single person, you should use the private chat.
If your city council uses Hoplr, you can also make your Idea visible to them. That way they also stay informed about what is happening in the neighbourhood.