Your first layer to your pages will either be watercolor or black lines to divide your page. These are great ways to conquer the white page and to give you variety when you continue work on your pages.
I recommend that you do all the watercolor exercises, so that you can learn what you enjoy doing... so you can do more of that.
If you are making the handmade journal, then I recommend creating several 9x12" sheets of paper. Have some with different watercolors on each side. Some pages to have the lines from "divide the page" on both sides. And some pages with one side of watercolor and one side of lines. Keep it random. Having these sheets will give you the most variety and options when we make the handmade journals. You may not use all of the sheets to make the books and that's ok. You can also make several books, if you want.
If you are working in a pre-made art journal, then paint some pages with watercolor and divide other pages with lines. Think of these exercises as prep for further doodling.
You may find that you really enjoy some of the watercolor exercises. They are designed to be quick. Feel free to come back to this section as often as you'd like to simply enjoy watercoloring a page for a few minutes.