When creating a plugin for any open source system it is always good to check if what you have in mind is already created and also a good starting point is to check out other plugins already made for the system. My first Frog CMS plugin came about while browsing the forum and seeing a discussion about tags. The original plugin was created by someone else but in its early stage I joined in the thread and improved on what was originally created. In the end Tagger was the product of the that thread.
Tagger was necessary for the original "The Hub" that was built on Frog CMS, the articles that were created on there needed a way to organise them. Now lets get started with how to approach making plugins for Frog CMS.
Think of what you want the plugin to achieve and start thing about how it will work.
Search the forum and contribution list of Frog CMS to see if a plugin with the description you have provided is already available.
Always try and start from somewhere. Since version 0.9.5 of Frog CMS there is now included a Skeleton Plugin to give you a base to start from.
4. Learn from others
Look at other plugins even if not related to what you are trying to achieve and learn the code, also copy code if necessary, well ask the author of the plugin first.
Ask questions on the forum, irc and network with other plugin developers. The Frog CMS community is quite helpful, there is not harm in asking. I myself normally check out the forum ever so often.
6. Release cycle
Start making versions of the plugin and have a good way of releasing it. I normally bring it through stages, so I start off with Alpha, Beta and then on to Stable release.
Write clear documentation. Documentation should only be necessary for referencing, the plugin should be intuative enough for a user to understand. Documentation is a must.
Get some users to test you plugin. This is normally a easy task if the plugin is something that will benefit others.
9. Feedback is good
Take all feedback as they come, Pros and Cons are good from a user's perspective as they will help to evolve the plugin even more in later developments.
10. Have a dedicated page to the plugin
Create a page on your website for the plugin and keep the users up-to-date with what going on with the plugin. I am no means a expert at creating plugins for Frog CMS and not all my plugins were successful, but I do have really successful ideas that the community is still using even now. Creating new plugins for Frog CMS is fun but always remember to make what is necessary and not just for making it sake. Be challenged and create new stuff, keep within the boundries of the CMS and never change core files to make your plugin work. I think if you have to change the core for your plugin to work, your plugin is poorly designed and I have no use for it.