LOL, IolantheAlias. So true, so true.
Welcome KenJ.
If you want to dip your toe in the water, you could start with a short story at some point in your plan. I rely heavily on feedback in this forum. Be warned that once you start writing, your plot may change of its own volition. We blame the detours and changes on our "Muse." When that happens, my advice is to go with the flow and listen to your muse.
Size really doesn't matter, but when I'm writing I try to make 14 word pages a chapter (roughly 5,500 words). The need of filling the space actually sparks ideas for me.
Since you are writing about Alt Clark, you can use that episode as a springboard and get ideas there. It also then ties it to the LnC world we are familiar with.
When you have something done, ask for a beta reader. Find them here:
Beta Readers Good Luck!
Artemis
Oh, if you can't find your post here tomorrow, look in "Fanfic Related". I have a suspicion LabRat will move it.