Depends on the type of 6 marker
For RPA 6 markers make sure to include stuff about working scientifically (errors, variables, accuracy, repeatability) and write steps clearly in order
For evaluate 6 markers include 2 pros, 2 cons, and one reasoned conclusion
For explain 6 markers I think 3 explained points should be fine (roughly 3 sentences on each)
For compare 6 markers have at least 2 similarities and 2 differences (ideally 3 of each though if you have time)
Make sure you know your command words (describe, explain, evaluate, compare)
Also using bullet points can help a lot
Maybe planning your answers during revision time for different ones so its easier to think through an answer in an exam before writing- i used this quite a lot
Remember to use SPaG and key terminology (science specific)
Using a highlighter to find the command words and if its got a table to highlight pros and cons can help some people
Planning your answers during the exams on the paper at the back of the booklet could be useful if you’re likely to forget what you want to write but make sure doing this doesn’t make you run out of time