You can use bar soap or dishwashing soap. Make sure the ring is evenly coated before sliding it on the mandrel. A ring mandrel is a graduated metal cone, which is used to size rings. You can easily obtain them from general online retailers. Your strikes should be gentle yet firm. Strike at a downward angle; you’re essentially trying to move the ring further down the mandrel. Make sure to turn the ring as you strike, to evenly stretch it.  If you have access to it, use a vice to secure the mandrel. This will make this step much easier. If you only have access to a carpenter’s hammer, you should cover the ring with a soft cloth to prevent damage to the band. If it’s still too tight, you can repeat the process, placing the ring on the mandrel and hammering until it fits. Keep in mind this method can only stretch a ring about half a size. If the ring is struck, you can strike upwards with the mallet to dislodge it.

Summary:
Lubricate the ring with soap and slide it onto a ring mandrel. Gently tap the ring with a wooden mallet or jeweller’s hammer. Remove the ring from the mandrel and try it on.