A Level NEA Coasts

I'm doing my Geography NEA, "Investigating the littoral zone at Hengistbury Head" and one of my primary data collections is going to be Sediment Size at three different sites along that stretch of coast. The sub question this would be answering is "How does sediment size, shape, and angularity vary across the Hengistbury Head coastline?". How exactly do I collect the samples for each site? I'm not sure if I should do systematic or random and if I do random then how would I carry this out?