To add a third dimension to the flat journal page I used medium artist gel over thick and thin stencils. The "thick" ones are made out of acrylic & generally intended as "drawing helps" and include lettering stencils & stencils with small simple shapes.
Use a plastic tool to fill in the stencil & scrape it smooth across the top before carefully lifting stencil. Let it dry overnight. To make the production (of waiting for it to dry) more worthwhile I did one in each of my four current art journals.
Stencil for lettering are "thick" so it fills up with a lot more artist gel. These stencils also slide around less as you're working with them.
The super-thin stencils found in the stenciling section of an art store are generally intended for use on a wall or with mixed media art. Generally you'd spray the back with repositionable mounting adhesive so they don't slide around. I didn't spray; I simply was as careful as possible to ensure the stencil didn't slip. The amount of medium required is much less; the end result "thin" on the journal page.
Once the gel dries (clear) you can paint and stamp and collage around/over as desired.