How to write a sponsorship letter to send the british embassy?

I need to write a sponsorship letter for my girl friend and invite here to visit me in Britain for a two week holiday and i need to send a letter of sponsorship to the british embassy in belarus .I am self employedand and willing to cover all her travel expenses aswell as the living expenses while she stays with me on her visit .But i don't know what to write ( how it would look and what would i put in the letter ) So need a proper sponsorship letter and i am really hoping that someone would help me?? please i have a problem with this....