There are many resources available for developing your proposal writing skills.
Here are some of the best:
A Practical Guide for Writing Proposals
Foundation Center Learning Lab -- Virtual Classroom
The Grantmanship Center YouTube Channel
The Grantsmanship Center
Grantwriting Tools for Non-profit Organizations
Guide for Writing a Funding Proposal
S. Joseph Levine, Michigan State University
Guidebook for New Principal Investigators
Canadian Institutes of Health Research
Small Business High Technology Institute
Proposal Writing Short Course
Geever, Jane C. The Foundation Center’s Guide to Proposal Writing, 6th edition. New York: The Foundation Center, 2012.
Browning, Beverly A. Grant Writing for Dummies, 5th edition. New York: Wiley Publishing, 2014.