The Call for Proposals and the Spot User's Guide, both available on the SSC website, provide important information on many things relevant for proposal submission. The SSC provides a mandatory template (available on the same web page as the Call itself) for observers to use in preparing their PDF proposal attachment. Proposers must use the template - there are no exceptions. Note that for Cycle-4, the inclusions of An Observation Summary Table is mandatory. If you read no other documentation be sure to Read the template instructions and follow them. That is the best advice we can give for submitting a successful proposal!
A Step-by-step proposal submission guide is available online here:
That page has many embedded links to other resources. The content from this website is repeated below, along with some hints and tips for creating good proposals.