The total mark allocation is slightly complicated, but basically each of the labs is 3.125 marks (25/8). Of this, 20% (I.e. 0.625 marks) is from the pre-lab work. From this, half (0.3125 marks) is given for submitted the pre-lab questions on time (At least 10 min before the lab), and the other 0.3125 marks is from your actual answers. The pre-labs were 2-4 very basic questions, usually multiple choice, on the content of the prac and/or the basic physics theory behind it.
In the actual labs you'd work in groups of 2-3 to complete the experiment, collect results/data and write a report. Definitely read the pages in the handbook on the relevant lab before, but don't stress if you don't understand everything as your demonstrator will talk you through it at the start and show the group how everything works. The handbook also contains an example prac which you can base your report structure on, as well as what to do and key points to note down as you do each particular experiment.
As someone who struggled with the practical report writing side of science in high-school, it got easier as the semester progressed, even though I often felt like I was missing stuff and/or not doing it right. Also, half the marks were basically participation marks (For following safety procedures, working well as a team ect.), so if you turn up and do your best/engaged with it, you'll probably get a decent mark regardless. I thought I completely failed the first lab (I struggled with what to write, barely talked about the physics concepts, and my group got 30 m/s^2 something for Earth's gravity!
) and ended up with 70% for the actual lab work (I.e. Not including the pre-lab stuff), which only improved from there. Prac content didn't always relate to what was currently being taught it lectures, but everything you needed to know is provided in the lab book and is fairly straightforward.