Any time you want to take an idea into your employer to get support for anything, it is always an involved process if you want it to work. I do a lot of proposals at my company, it is part of what I do - research vendors and investments for the owner of the company. The first thing you'll have to do is put together a detailed profile of the particular charity. Gather as much information as you can, good and bad, so you can be sure you are presenting an appropriate organization that has some relevance to your employer. The most important thing here is this: KNOW YOUR COMPANY'S GOALS!!!!! If it is a small business, try a charity that is localized to your community; if it is a multi-state or multi-national, a larger charitable organization might be acceptable.
The main thing you want to keep in mind is that most business owners don't just do things without wanting to get some benefit in return. So, part of that proposal should state very clearly the advantages to the owner and the business that will result from them accepting your idea. It helps if you have a good idea of the financial situation of the company - I do the books where I work, so always have access to our bottom line. Let me give you an example....
You want your business to donate money to a local homeless shelter that you are involved in. Sending an email to your boss and asking them to give in the company's name will probably be ignored. However, putting together a presentation, sitting down at a meeting and presenting a well-thought-out plan to sponsor a community event, with a clear method of organization and execution, while stating the advantages to the business will probably at least get the idea considered.
It is not an easy thing you are hoping to do, in the world of business, and you should be prepared to have any project you might propose dropped in your lap to organize and execute!!!! Never underestimate a boss's ability to delegate!!!!!
If you need any more help, just let me know and I will give you what advice I have. Remember to make everything relevant to the business where you make your proposal!! Good luck!