Lay out the poster board on the floor (or ground if working outside) to measure 6 feet long. Tape pieces together so you create one long line. Make sure you tape the pieces on both sides, as the child will be stepping on the poster board and you don't want it to rip easily. Secure the poster board to the surface with packaging tape.
Use the ruler to measure out spaces between the numbers on the number line. For basic addition and subtraction, keep in mind that this is not being used for measurement, so a replica of a ruler is not necessary. You just want the numbers evenly spaced to help your child visualize the number line. Be sure to make the numbers far enough apart that your child can easily step from one number to the next.
Create a number line for practicing addition and subtraction with integers (- and +) by finding the center of the poster board to mark as "0," and write the numbers accordingly. Be sure to place out the numbers on each side of "0" evenly, meaning if you end with "-10" on the left side, you should end with "10" on the right side. This helps the child visualize the number line better.
Create a number line for practicing addition and subtraction with fractions (1/2 + 1/2) by duplicating a ruler. Just be sure to write the fractions on the number line for your child to see, so he doesn't have to guess which line is which fraction. Of course, if your child is advanced, not writing in all of the fractions could make a good thinking exercise in addition to the problem he is working on.