@Mark *****
nice tips overhere. Although I am from the Netherlands (Schengen), I think it's the same for every country; they want to be sure she goes back to Thailand after 90 days. I also wrote a letter in which I said I would take care of her and also would take care of her return. With my phone number and that they (Embassy, now also VSB Global) could phone me
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Then, I also needed a Thai healthcare insurance for her and a stamped certificate of guarantee by my municipal office of my income and gave her place to sleep in my home. But long time ago and UK not schengen. I started the same like you by asking and gathering info. Even got the tip that she must dress appropiate when going for the application π I did everything myself. An agent will normaly ask you the same proof and papers. Good luck !!