Always fly your spinnaker before going afloat. To check an asymmetric, start at the tack, run up the luff to the head, then down the leech to the clew. Check that the spinnaker sheets go round the jib and are over the retrieval line.
Once hoisted, the sail develops a lot of power, so steering is the key. The steering must balance the forces on the boat. The crew cannot move into the boat until the sideways force is decreased by steering the boat away from the wind (see power zone diagram below).