There will be an agent downstairs in Trendy office that will help you fill in the online for for a fee. Be prepared for her parents details, such as Thai ID and dates of birth, her full travel abroad history, all hers kids names and DOB, give an estimated day of arrival in UK and an estimated day of leaving, if she has money in the bank .. copy of her bank book , statements etc.
They will want to know total cost of the trip … including air fares and spending money. Say you are paying for everything .. so if cost is £2500 estimate then you would say you are sponsoring the trip for £3000.
If you have a healthy bank balance submit your own bank statements with the application.