There are oodles of ways to make homemade paste for paper mache.
one way is to mix 1 part flour and 2 parts water, to make a substance similar to thick glue. Add a smidge of salt to prevent molding if you live in a humid area.
you can also use elmers glue watered down in a bowl, dip your strips in there, and let them get a little gooey, and put them on the belly, smooth them out and keep applying them
basically the paper part will always be the same, but the glue can vary. almost anything that is sticky and can be used, as long as its not full of chemicals to harm the baby :) Just stay away from substances that mold, melt, are greasy, or dry too fast. you will want to finish the belly before it all drys out on you.
(you may want to grease up your belly with lotion or vaseline, not too much, rub it all in as if its lotion) this way you wont pull off any hair, and it will make a barrier to protect your skin from absorbing the paste. If you use too much though, the water in the paste wont bond to your belly because oil and water arent the best of friends and that may cause your paper to slide off and shift out of place. experiment a little to see what combos work best