Generally I add them to my current art journal the same day I've made them; or at the beginning of my next art session.
I use a Rotatrim paper cutter to cut to a size a bit smaller than my journal page; fold it in half (or where I want the seam to be). Then I use an ATG double-stick tape gun to tape the edges of what will go on the right side of the jounal page.
Shove the taped/folded edge into the seam of the journal & gently roll it down. Then use the ATG gun to tape the back of the left half of the gel print and gently close the journal page.