I have got 3 visas for uk for my girlfriend. First a 1 year using an agent. In year 2 I did it myself and got a 2 year visa for her. Then on the next application did a five year. That’s just ran out. I’m applying too. Things have changed a bit.
Most of the docs and photos you need are downloaded on to the uk gov website but some must be with her when she goes in person at VFS Bangkok for Biometrics.
Any tourist visa is valid for a year but they can’t stay more than 6 months in that year.
You need to show a reason she’ll return and you need to show your relationship is real- phots, hotel bookings, phots with family etc
A letter from you explaining the relationship and why the visit and the fact you’ll pay. Photos of where you live/ will stay.
I’d make sure she can provide a bank statement with a grand in. I’d say she is only coming for a month- you book flight after visa for longer.
Get familiar with docs required for both web sites and start accumulating everything. Make a list. You’ll know when you are close. Good luck