If you have to do it again, it can be simple. Before you place the new valve into position, use a liberal amount of grease or Vaseline to keep the seals positioned correctly (peanut butter in an emergency). What usually gives most people a problem is that the seals get dislodged while trying to squeeze the assembly back into place. Typically, the grease will do the trick. If you're still having issues, take a cheap plastic mat (plastic milk carton in an emergency) and cut it into two pieces approximately 5" x 12". Place the mat on both sides of the valve, protecting the rubber seals. Insert the new valve into position and once lined up, pull the plastic mats out while holding the valve in place.