you need to write her an invitation letter, explaning how long you have know each other, and how did you guys meet and stuff, just very briefly. in the inviation letter, also include her personal details, bday, passport number, address and etc, and same for you. also emphasis the length of the visit, what you guys will be doing e.g. travelling or staying in London and you will be paying all the cost, and also you will make sure she leave uk at end of her holiday. then attached your passport and uk visa photocopy, your bank statment, maybe a letter from your school providng your student status.
for you gf, she needs a letter from her school proving her student status and her holiday. if you are paying the cost, i don’t think you even need her bank statement, but if you got it, there is no harm to put it in. i will reco妹妹end 80-100 pound a day. or you can say you will be providing accomodation in the letter, so you can just prepare 50 pound a day.
my bf had 3, 6 months mulitple entry visa already. should be very straight forward. very happy to help if you need any further info.
hope she can make it, ldr is not easy. good luck.
我以为是这样的，你再确认一下，查一下国际的英国签证官网就能了。pleasure to help，祝你所有顺利。