Make or purchase a form for the concrete. The concrete block must be poured in a form. The form will determine the shape and dimensions of the concrete block. The form can be any shallow container that is sturdy enough to hold the concrete until it dries and that you do not mind destroying in order to remove the concrete block. A suitable form can be designed by nailing four 1 by 4 boards to form a bottomless box with the appropriate dimensions, or by choosing a sturdy, appropriately sized, cardboard box, such as a shoe box top.
Mix the concrete by adding water to the concrete mix in a bucket that you do not mind destroying. The amount of water to add will be listed on the package of the concrete mix that you purchase.
Pour the concrete mix into the form. A block exceeding two inches in thickness will be very heavy, whereas a block less than one inch thick may be fragile.
Smooth the concrete by moving a trowel or flat board over the freshly poured slab.
Place or impress the desired shells, animal bones, leaves, twigs or pieces of coral in the concrete. Making an impression of the object will leave an outline in the concrete, whereas placing the object in the concrete will encase the object in the block when it dries.
Wait for the concrete to dry. The recommended drying time will be listed on the bag of concrete mix that you purchase.
Paint the "fossils" with brown or earth-tone paint to achieve an authentic appearance.
Turn the form over and remove the concrete block. If you used a cardboard form, it may be necessary to peel the cardboard away from the concrete block. If you used a wooden form, it may be necessary to pry the boards apart or remove the nails from the boards.