It came to my attention that lady Rachel is not on CIA book, smart move Rachel 
I have to be there but …yes..long story , under fake name of course but …never mind…point is heard from ..someone….you made a somewhat wacky start, like me, so , in short best not to make it complicated that is the main thing, keep it simple, it is worth a new bag of plaster of Paris aka gypsum….that is a must be, not an old one from years ago from the gardenhouse …. mix ratio is on the bag but we usually do one on one, one ration plaster, same ratio water,
do not make it more complicated, at least not in the start, one can do that later.
Later….
Sills have to be build up….step by step…
You will get there, they have a pleasant feeling
You are like me, want to do ten recipes in one go , that usually does not end well…
We love all energy junkies
Shall I say welcome to the tribe.
Do not doubt yourself that is the task of others 
Good luck
