Use a 20 inch bmx inner inflated so it holds it's shape, place the inner tube in the gap between the tyre bead and the rim so it fill the gap, inflate the tyre and pull the inner tube out as the bead of the tyre reaches the rim of the wheel, this is normally very successfully to get the tyre to seal, once the tyre is holding pressure continue to inflate until the beads seat (please be cautious of over inflating the tyre especially if it has any visible damage)
Alternatively a bead seater is a great investment.