First, you need to achieve the rank of Traveller. This is done simply by completing a plot in the small plots world (/spawn, /plot auto) to a good enough standard. You can read a more detailed guide on that on these forums (TBA)
Once you have Traveller, simply head over to this thread, read the Requirements, and post your in-game name on the thread.
You’ll be promoted to the rank of Trial Builder as soon as an Administrator can do so, and at that point your trial starts.
It is vital you keep a log of all the project work you have done in the form of screenshotting everything you do for Project Leaders on their projects. Keep them all on a folder on your PC or something. Then, once your trial is complete, you will upload them to a file sharing website such as Imgur.
After this, you should post your album in a new thread in the “Builder Applications” category. In the thread you should include the date of your promotion to Trial Builder, and a list of all the projects you have worked on.
The standard length is 1 month. If you’ve worked on a lot of projects in your first 2 weeks, you may submit your application after 2 weeks. If there haven’t been many projects to work on in your first month, or you simply have not had the time, you may talk to a Creator about extending your trial period.