As the dealine for Cycle 1 General Observations is approaching (2018 April 6th – see here), I have put up a cookbook specific to solar system observations with NIRSpec. Owing to the GTO and ERS proposals, I have been through the process of using the ETC (exposure time calculator) and APT (astronomer proposal tool) extensively (albeit for NIRSpec), and I know about the learning curve… If you have a good science idea, this might actually take quite some time to translate it into a JWST proposal.
In this cookbook (download PDF – upcoming) or the related pages I have been putting up (here), you will find some generic information on NIRSpec, JWST features relevant to moving targets, and quick guides to the ETC and APT. However, please note that the official STSci/NASA webpages should always be considered as your bible – be sure to actually check them, as up-to-date information will always be found there first. I only mean to provide you with a basic guideline to get you started on your proposal.