A footer or footing is the base of a foundation and block walls are only as strong as the footings that are under them. Before you start building with concrete blocks, you need to create a solid and immovable base that can support the substantial weight they represent. Ideally, a footing should be poured directly onto bedrock. At the least, the footing needs to be in a trench that has been dug about 12 inches below the frostline, to keep the ground's freezing and thawing from moving the wall.
Dig a trench that is 12 inches wider than the footing you plan to pour. If you dig down to bedrock, clean off the rock with a wire brush and a hose. The cement will create a solid bond when poured on bare rock. Otherwise, firmly tamp down the earth at the bottom of the trench.
Build a form that will contain the cement when you pour it. Place two 2-by-6s on edge and separate them by the desired width of the footing. Secure the 2-by-6s by driving in strips of 1-by-3-inch pieces of wood on their outside faces. If your form is on bedrock, secure them by screwing cross pieces on across the tops of the 2-by-6s, then wedging pieces of wood between their outside faces and the sides of the trench. If you want to reinforce the strength of the foundational footings, lay pieces of rebar in the form, suspended so they will be in the middle of the completed concrete footing.
Mix the cement yourself in a wheelbarrow, or call in a ready-mix truck for larger jobs. Pour the cement into the forms as gently as possible to avoid moving or damaging the forms.
Smooth the top of the cement by resting a board flat across the upper edges of the forms and moving it across the cement in a sawing motion.
Leave the foundational footing to dry thoroughly, then remove the wooden forms before stacking the block wall on top of it.