Having identified a project and an issue you would like to work on it is a good idea to introduce yourself to the project community and outline your plans for tackling the issue. This means the community know you are working on the issue and gives the community an early opportunity to help course correct your proposal if necessary (which can save lots of time).
Here's an example of what your first most might look like:
SUBJECT: A proposal for Skinnable widgets (WOOKIE-131) Hi, I'm new to the Wookie project but I'd like to tackle the "Skinnable
widgets" issue (WOOKIE-131, see  ). Before I launch into implementing this I'd like to run my ideas past you to make sure I'm not missing something important.
If I understand the issue correctly we need to define the structure of a
minimal widget. For example:
<div id="wookie-widget"> <div id="wookie-toolbar">This is where the toobar will be</div> <div id="wookie-content">This is where the content will be</div> <div id="wookie-footer">This is where the footer will be</div> </div> We also need to provide a default CSS for rendering this as, for example: +-----------------------+ |TOOLBAR | +-----------------------+ | | |CONTENT | | | +-----------------------+ |FOOTER | +-----------------------+ This will need to be documented and all widgets should be changed to
conform to this format.
We would need to document this on the Wookie website and also document how
to extend this.
Once all this is in place I would like to start extending it to provide
docuemnted CSS identifiers to be used in common cases, for example, we may want to define "wookie-navigation" as being used for a common navigation menu. We can then provide various CSS alternatives for rending this as a vertical navigation menu or drop down menu, or whatever.
Am I on the right track? Are there any important points I'm missing? 
There are two main advnatages to sending a mail like this. Firstly, you force yourself to think carefullly about what is needed and whether you have the skills (and thus where you will need help). Secondly, it allows people in the community to provide intput. For example, in the above case the community would probably point out that there is a widget template system that needs to be modified to ensure this format is used consistently.