You will need several things, some paste flux, some lead bars, an acid brush, wooden spoons dipped in parafin wax, and an acetylene torch. Propane and propylene are not acceptable as they cannot heat as quickly as acetylene so the heat will spread to surrounding areas and cause increased warpage. You can buy paste flux and thin lead bars at some hardware stores. If you were to buy a kit for filling with lead, it would come with special paddles for pushing the lead around while it is semi-solid, but wooden spoons dipped in parafin wax are much cheaper. To actually apply the lead, you must first tin the part. You apply the flux to the area to be soldered with the acid brush, then heat it with the torch. Pass the lead bar over the surface frequently and when it starts to flow onto the surface, spread it around until you have a thin film covering the entire surface to be soldered. Be very carful while doing this because you are very close to the temperatures at which the steel will start to melt. Once you have the surface "tinned" let it cool. Patience is you're biggest friend when using heat around sheet metal. Now that the surface is tinned, you can basically melt the bar of solder into the dent, then add a little heat to the dent to get the solder to stick. Use the wooden spoon to push it around and get the shape you want. The solder will maintain a butter like consistency as it cools some. If you are having difficulty getting it the way you want it, stop and let it cool. If you spend too much time in one place it will warp. Once you've got a big blob of solder layed out, take a file with a very open tooth (not quite a rasp) and start working the lead into its final shap. An open tooth file is a must because a smooth file will load up very quickly.