First make sure you understand the criteria for 6 marks. To obtain O.8a requires a more sophisticated approach.
If you use simple equipment, you must use it correctly and get the best from it.
Two people are given the same equipment to carry out the same experiment which involves "simple" measuring with a ruler. One person gets excellent results the other poor results. Why? The skillful person will set up the apparatus so that measurements can be made easily and accurately. They will probably take down, adjust and set up the apparatus several times to find out the best arrangement. They will take care to measure correctly and avoid errors, and they will check their results as they go along. Ah, but this will take a lot longer, you say. They will probably take a little bit longer, but not much. Their skills are much greater because they make the most of the time available and don't wait to get started. The person with not such good results will probably be thinking (in their bit of extra time) that when it comes to the Big One they will spend a bit more time and effort on it so their results are just as good. Unfortunately things don't work that way. You don't become a good experimental scientist by wishing about it, but by practising it. You wouldn't claim to become a decathlete by watching the Olympics on television. That takes lots of practice at lots of different skills. Investigative science is the same. You learn best by doing it, not just watching someone else do it. (It also helps to have a good coach, or teacher!)
Some apparatus (obviously) requires more skill than others. A micrometer screw gauge used to measure the diameter of a wire to one hundredth of a millimetre is more difficult to use than a ruler.
Evidence is reliable when you know it is correct within the accuracy of your method. Somebody else carrying out the same experiment would get the same results.
If each repeat is quick to carry out you will only have "good" results in your final results table. For 8 marks you are expected to spot errors as you go along and correct them by taking more results. You are expected to select the results you include and not include anything that is irrelevant.
For a longer experiment, lets say where you have to take readings every 20 seconds for ten minutes, it would not matter so much if one of the results was wrong. Suppose you were distracted and wrote down an incorrect value. It would be alright to put a small note such as "no measurement recorded" in the table. One missing point will not make much difference to the graph you will get.
You must have an adequate range of measurements to fully test out your prediction. If the range is too narrow your conclusion will be limited to saying what happens in that narrow range.
Usually you start off by trying out as large a range as is practical. Once you have sorted out a sensible upper and lower limit you divide it into equal steps and take your results. Plot the results on a rough graph as soon as you have them. If the line is straight or slightly curved you are all right. If the graph curves sharply, or changes quickly, you often find that there are big gaps between some of the plotted points. You will need to take extra results to fill in the gaps. If you do not do this your line of best fit will turn into a guess of best fit and not get you 8 marks.
NOTE: If you carried out preliminary work you will probably already have a good idea of the best range of measurements to take. Don't forget that it is never too late to revise your plan if you are going to get better results. Any changes you make to your procedure as you obtain your evidence is counted as an improvement to your plan (and may add to your Planning mark). Your plan should not be formally assessed by your teacher before you carry out the practical work. It should, however, be checked to ensure that it is safe.
Don't forget to note down any relevant observations about the results. Also make a note of any difficulties or problems encountered, or anything special or unexpected which occurs. Often you are too busy to make detailed notes while you are doing an experiment. Make sure you fill out your rough notes with more details while they are still fresh in your mind. i.e. later on the same day!
The best scientists NEVER cheat with their results