What happens during a solo checkride and how do I put myself in the best position to be successful?
It's pretty simple.  You will be asked to demonstrate everything you've learned.  The Grade II is evaluating you and is also evaluating your instructor's teaching skills.  The two hour block probably includes extra time allowance if the check instructor wants to give you remedial or supplemental training before signing you off (assuming you are acceptable), and the actual flight may be the full 2 hours or quite a lot less.

Just relax and do what you're told to the best of your ability and don't worry about it. To best prepare, go through all the things that you may have to recall from memory, especially emergency memory items or sequences or mental check lists, and drill drill drill that stuff on the ground while at home or while driving until you can't stand it any more, then do it some more.  You want to be able to spit out procedures or numbers automatically without thinking, under pressure, with distractions, and to do that it just takes repetition (have a friend shove you and yell at you while you recite everything - you'll know right away if you have things internalized sufficiently).

I cannot stress that aspect enough.  If you have everything that needs to be memorized fully burned in to your brain beforehand, it will massively reduce the stress of the flight itself and you'll find you are better able to free your brain to concentrate on flying and it's only a matter of having the physical and judgement skills in place or not.

The other thing to add is that instructors and examiners love students who show up well prepared, who have obviously taken a lot of time and effort to learn what they need to learn, who can spit out answers to verbal questions immediately without having to mull the question over (to a reasonable degree).  The result is that they will tend to cut you a bit of slack on little screw ups you make during the evaluation or test if it's obvious you are putting in the work, and obviously, aren't dangerous.  In the end they just need to be satisfied you won't kill yourself, and a lot of that comes from a general feeling of comfort the examiner gets from the overall experience of flying with you and knowing what you have put into it.