Things you should do:
Read all the set texts.
Make summaries of all the set texts.
Read broadly about the issues discussed and come up with your own viewpoint.
This is pretty much right, but I'd add a few things.
Reading the texts in Philo is not really like reading your English texts over the holidays. I know when I was reading Emma for Lit I was only partially paying attention (it was summer) which was fine because you just needed to have an idea of what was going on so you weren't going completely from scratch when we got to it in class.
Reading the texts for Philo is useless if you're skimming or not concentrating completely. If you don't understand a paragraph (which will happen), read it again two or three times (after 3 cut your losses and come back to it when you get to it in the course).
I actually think that's more valuable than making summaries, which I didn't do until term 3 holidays and only because I'd forgotten a lot of stuff from terms 1 and 2. The risk with making summaries in Philosophy is that for a lot of the texts a sequential summary of it will not obviously give you the argument which is the main thing you need to take out of the texts for this subject. Hume, for example, one of the Unit 4 Science and Knowledge philosophers, makes the same point in about three different ways across the course of the set text - just summarising would be pretty useless.
I'd suggest instead just getting to the end of the text and thinking to yourself (or writing if you want evidence of your own productivity) two things - "what is this philosopher's main point?" and "how did they reach that conclusion?" (or "what is their argument for that point?"). If you can answer the first question and have a decent go at the second then you're pretty well-placed at this stage.