Here is one of my favorite techniques for making borders. I start with a page in my journal that I've painted with a variety of layers.
Then I make color copies of a few pages from another journal. Use pages that have a lot of detail and color. Cut or tear long 1" strips from these copies.
I glued the strips around the edge of my page. At this point you wouldn't need to add anything else, the colored strips look as though you have tediously painted all the detail and color!
However, I did want more contrast and added the black collage paper.
And to finish it off, I used a white Sharpie marker to draw a frame within the border.
Now it feels done.
Simple and amazingly quick!