Head to your local hardware store for the following 3/4" pipe and fittings:
2 ten foot lengths of PVC
3 end caps
1 threaded female hose connector (slip fit)
2 elbow joints (90 degree)
2 T connections
Time to cut.
You will need the following sections and sizes:
2 at 5 feet
1 at 4 feet
4 at 18 inches
Cut one of the 10' lengths in half so that you have two 5' sections. Then, cut a 4' section from the second one. Cut the remaining 6' section in half and then each of the 3' sections in half.
Time to Drill.
We want all the sprinkler holes pointing more or less in the same direction, so start out by making a center line on the three longest pieces of pipe. Hold two pieces of pipe next to one another and use one as a guide to mark the other. The angle you hold your pen/pencil at isn't important, just keep it the same and you'll end up with a straight line. I found that a 5/32" drill bit was perfect for the holes we wanted.
Make sure there are no PVC chips or other debris in the pipes before assembly. Fit the drilled pipes together to form a "U" shape with all the drilled holes facing inward. Once you're satisfied glue up the "U", making sure the holes stay facing inward.
Time to Assemble.
Now that the "U" is glued, we need a base to stand it up on. Assemble the "U" and the base without glue. Make sure the base pieces are parallel with each other, and perpendicular to the plane of the "U". You can place the base pieces on either edge of a sidewalk for a handy reference before gluing it up. Line the "T" pieces up with each side of a crack, then make the base pieces parallel to the edges of the sidewalk and you're ready for the final glue.
Once the glue is dry, you can attach your hose and try it out.