Develop the concept. Write out a logline (one sentence description), synopsis (one page about the show) and treatment (detailed breakdown of the show and it's structure). Rewrite these until they are perfect and you are sure the concept is will be bought.
Talk with the producers and director to figure out the complete layout and structure of the show. Determine whether there will be prizes awarded each time, who presents the challenge or task, when the cameras will be rolling and so forth.
Interview the sponsoring or partnering organizations and associations, and develop challenges or tasks for each week of the show. Plot these out as much as you can, but leave things out to any possibility, as with a reality show anything can change at a moment's notice and you have to be prepared for that.
Open Final Draft and create a structure for the show with act breaks, host introduction segments, suggested cutaways, good places to insert interviews or confession moments and so on. Make it a bare bones structure that you can work with from week to week so it will simply be a matter of pasting things in quickly later.
Fill in all the gaps as the week's episode is filmed. Take notes during filming of particular scenes that need to be included. Then write the voice-over narration that will be used to connect moments from the day or recap what happened in the previous episode or preview next week's episode. Also write out any speaking parts for the host where they will be speaking to the contestants or the camera directly.