FoundationCenter.org, Sophia.org and CapturePlanning.com all publish online tutorials for writing proposals, as of 2015. Other online sources of proposal-writing tutorials include Instructional Solutions and The NEA Foundation.
The Foundation Center's tutorial walks the user through the entire process of writing a grant proposal. It starts with how to gather information and gives descriptions of the components of a proposal. It also gives in-depth instructions on how to write each part of the proposal letter.
The Sophia tutorial includes written descriptions and instructions as well as video presentations for each step of the writing process. CapturePlanning.com provides tutorials and links to articles that give advice on writing proposals.