Comment
The simplest way to help build an episode plan is just to offer your feedback or suggestions. Are the contributors reliable? Does it sound like an interesting idea to you? Can you think of anyone else who should be featured, or other ideas which it would be good to add to the show? What do you think of the points raised so far in the discussion about this plan?
Use the discussion tab (second one along) to comment on an episode. This keeps the discussion separate from the plan itself. Since everyone is sharing a single page for discussion, remember to add you name below what your comments. Start with the episode plans in the Release Schedule if you're not sure where to begin.
Content
To qualify for Beta grade (production ready), episodes need nearly 2 hours (or more) of content, so the main way to improve an episode in this state is to find suitable content. This should be added to the episode plan as explained below.
Editing
Understanding the Episode Template will help you contribute stuff effectively. If you've found extra content for an episode plan, you should add it either to the |content= section, or the |possible_content= section, depending on how confident you feel that it will suit. Similarly, update the |contributors= or |possible_contributors= to reflect the extra content.