What are the steps to follow in order to start a proposal ? More precisely, what should I check if I want my proposal to be successful ?
Looking at your profile I see you are relatively new to the Stack Exchange family. In short there is no one thing that make a proposed site successful or fail. The ability to communicate thoughts on the topic in a Q&A format is the number one criteria, but there are many variables for other criteria. What works for one topic may fail another topic.
The number one thing that makes an Area51 proposal work is the ability to bring in participants. All of the easy proposal to launch have been launched. Expect that you will need to do something to bring in the followers and participants for your proposal, even if the idea is GREAT, unless someone bring the people here, it will not succeed.
There is no cost (other than your time) to make a proposal here, so just do it. Test the waters and see what happens.