Some more details
Utter novice in rifles, but was taught carpentry by a cabinet maker. You want the iron HOT (not red hot, obviously, but hotter than a modern clothes iron) - an old fashioned, cast-iron iron you can heat on a gas ring is best. Alternatively use an appropriate piece if solid iron (with a smooth surface) clamped securely into something like a mole wrench. You will need to make a good seal with the wood for the steam to penetrate, so if you are working on a curved surface you will need to press at the right angle or use a matching shape of iron.
Multiple layers of wet blotting paper or - best of all - commercial paper towel (the individual ones, not the soft stuff off a roll) works better than fabric. Obviously anything containing synthetic fibres is a bad idea. If you're doing it right the iron will dry the paper in a few seconds, so careful you don't burn through and damage the wood. Press very firmly and watch you don't burn your hands on a jet of steam.
If you're unsure buy a small piece of walnut timber from ebay, or get a free sample from a worktop supplier, dent it with a hammer and get some practice in advance.
An oil finish won't help, and it will be lifted in any case if the steam penetrates beneath it successfully. Also remember to let the wood dry for a day or two before you re-oil - you have just re-inflated the wood fibres with water. If the wood fibres have actually been broken or cut then the process won't work so well.