Which institution do you prepare the proposal for? Considering this issue is highly important. Typically, for MSc and Ph.D. dissertations, there is a specific format for proposals. Before taking any actions, get a university-approved template to prepare the proposal based on it.
Consider the proper budget and time to write your proposal. In some cases, a lot of funding is needed. In other words, proposals can be considered by an organization that will provide benefits to the individual.
Having a good idea is the first step in drafting the proposal, but it should be noted that converting the idea to a proposal is very difficult.
In some cases, instead of the proposal, it is advisable to prepare the draft of your project, a simple proposal that is used for classroom research and is in fact a one-page proposal. This single-page proposal can be considered as the original proposal that does not need much time or budget, but it is also used for initial confirmation and presentation to the supervisor.