posy, initially you don't have to choose which is first and second, just make sure you have registered for both.
Once you have the results, then you will know better what to put on to the application form, which comes right at the end of the process.
Absolutely. You won't need an order of preference until you put in your CAF (common application form) to your local authority at the end of October. For now, all you need to do is start preparing her for the exam (English, maths, verbal reasoning and non-verbal reasoning). In the summer, you'll need to register for the test. Registration will open on Monday 1 June 2015 and close at noon on Friday 3 July 2015 - put these dates in your calendar now! I nearly missed registering last summer and would have been gutted if DD hadn't got a place just because I hadn't been organised!
You register to sit the test at whichever school you like, and then you can choose to "share" your results with some or all of the other GS. This means that your DD's score will be passed to these other schools, and she'll be considered for those schools equally alongside everyone who sat the test there and everyone else who "shared" with them. You have nothing to lose by ticking to "share" results with all 4 possible Gloucestershire grammar schools: Pate's (co-ed) and Denmark Road (on here often typed as HSFG), Ribston & Stroud High (all girls).
Your DD sits the test in September, a month or so later you get the results, which used to be a confusing mixture of scores and rankings but since last autumn have been just rankings for all Glos GS. You get an individual reply from each school, which compares your DD with all the others who sat there or shared with it. You either get told that your DD is "within the top 120" or whatever the PAN for that school is, or that she's not qualified, or that she's qualified but is outside the PAN and so can't be certain of a place and her ranking is X. They also supply information about the previous few years, and how far down the rankings they ended up offering places. This varies a lot between one school and another.
It is only after you get the results that you have to decide an order of preference. Because you are in Ross, you'll need to apply through Herefordshire, regardless of the locations of the schools you want. That means you only get 3 slots on your form, I think. You should fill in your three preferred state secondary schools (whether grammar or otherwise) in your true order of preference. The schools don't know what position you put them in on the form, and even if they did, it would be illegal for them to take that into account in deciding whether to give you a place. Your order of preference just tells the computers which place to give you if you are in the happy position of being eligible for a place at more than one of your preferred schools. You can also apply to indies if you like, but that's nothing to do with the CAF.
Hope that helps.