I think the easiest technique for altering a book might be to just get an old magazine and start cutting out letters, faces, bodies, or whatever else catches your attention. I keep a box of magazine cut-outs so that if I’m ever not in the mood to go flipping through and cutting, I can just go through my box and quickly pick a few out.
Also, keeping a box means I can travel with my box easily, which means I can journal whenever I want. Any kind of glue works for an altered book, but I personally like glue sticks because they’re not messy. Another great glue is scrapbooking glue, but that takes a little more time to dry, and it may make your magazine cut-outs crinkle.
Don’t be afraid to layer with your cut-outs. I, personally, really enjoy this, but again, don’t overthink it. You can lay out your cut-outs before gluing them if you want too. This will give you a better chance of avoiding any “mistakes,” but there really aren’t any mistakes in journaling or in art. This is more for the process than the outcome, so don’t get too stressed out and just let your creative juices flow. You’ll be happy that you did.