According to Family Handyman, you make concrete forms for sidewalks by lining the length of the planned sidewalk with 1 1/2-inch boards along the straight areas of the sidewalk, and flexible hardwood siding along curved portions. To prevent the forms from bulging under the weight of the concrete when it's poured, place stakes at regular intervals along the form's length to provide outside support.
Family Handyman advises the use of boards that are the height of the sidewalk. The spacing of supportive stakes should be at least 3 feet apart. If the forms extend underground, then pile soil against the forms to provide extra support. Use a plate compactor to pack the soil for a solid base. If the forms extend more than 6 inches in height, then move the stakes closer together to support the increased weight of the concrete. Plant vertical stakes against the forms to aid in support. Cut all of the stakes to the same height as the top of the forms to make finishing the poured concrete easier.
Concrete forms can be uniformly spaced by using a gauge stick, which is a board with blocks attached to the bottom to set the precise width of the sidewalk.