Center the union over the break in the pipe. Mark the ends of the solid portion of the union onto the pipe with a marker, if you are using a union with rubber boots. If you are using a PVC union, center the knurled portion of the union over the crack, and then mark each side of the knurled portion onto the pipes with the marker.
Cut out the section of pipe with a hacksaw or reciprocating saw. Use the marks as guides for where to cut. Cut as straight as possible.
Remove burrs created by cutting the pipe, by running a piece of sandpaper around the cut ends.
Clean the outside of both pipe ends with PVC cleaner, to remove any dirt that could cause a faulty seal.
Slide the pipes onto each end of the union. Then slide the pipes together as close as possible, to seat them inside the union.
Tighten the band clamps on each end of the union, if your union has rubber boots. You can use a screwdriver or nut driver to tighten the clamps. If using a PVC union, tighten the knurled potion of the union by turning it clockwise. As you turn the knurled portion of the union, the compression sleeves inside it grip around the pipe.