To designate my sections I use different coloured papers and just wrap them around the signature [the collection of folded pages] before I add them to my journal. I like to use papers I've painted as section dividers. But you could also use scrap booking paper.
Sometimes you can also insert a section divider within the folded pages to indicate a separation. In this example this list making template might be used just for art supplies and the one on the preceding page is a list for household items I need to purchase.
There are a lot of different ways to make dividers for your journal. I like to use a heavier weight of paper like a coverstock. The divider on the left was made by laying down squared sections of paint them using an old dried out marker to scratch out markings and symbols. The divider on the left was made by painting a paper with a base colour of turquoise, then using stamps and part of stamps in black and white paint, I stamped the images onto it. Both of these pages were then cut to the size of my journal pages and inserted.