Hi,
I was working as a buyer for more then ten years so I know how a good proposal looks like.

It is easiest to look at the proposal request if you have one. there should be a guideline on what they want and which information they require.

A Proposal is normaly devided in three parts.
1.Introduction which should outline your company and why you are good at what you do.
2. the services you are offering to the company and
3. your cost model

Dont write a huge company profile. Stick to the basics.
Make sure you are making the whole proposal easy to the eye and clear with no typing mistakes. (If you dont show quality in your proposal, you are not expected to deliver quality)
Best would be a cover page with a picture of the contact person and their contact details really, then half a page about your company and services and then 1/2 to 1 page about the services your proposing and then 1/2 about your cost model.
Make sure you show why your company is better than anybody else. Show your quality (like show that your other clients hardly ever moan but are happy with you, maybe even add a recommendation if you have one)

If you need any further information, or would like me to go through your proposal in detail with you I am happy to do so in a call.