The process of merging a project is simple, but it still needs to be described.
Quoting the main page on pwmirage.com:
[...] A community member makes changes in the editor, then with a single click of a button publishes those to broader audience. Next, the changes are discussed on the forums and eventually, when approved by the administrator, they land directly in game
Each project has an associated forum thread for discussions and new updates. On the project list, you can click either the project title or "Read more" button at the bottom to see the thread.
In order for the project to get merged it needs to be approved by at least two maintainers and the status is tracked with labels visible in top-right corner. The are 4 possible states:
Usually the first maintainer to review a project will give it the Approved label, then the second maintainer will change it to "Ready for testing". The project in this state will be verified on test server and if there are no problems it will be merged to the game. At any point before merging, any maintainer can change the status to "Needs changes", describe his concerns with the project, and potentially ask the author to update it. Then, the review process restarts.
You can search projects by status with the "Filter" button in the top right corner of the project list page.