The first thing you will want to do before you can install your new lead bend is to remove the old bend from the line. Once you have removed the old lead bend you will want to get all of your tools and the new lead bend into the area you are working. You will want to make sure that you have your smelting pot to melt the lead, some oakum, your lead working tools, your runner rope, enough lead to fill the joint and of course you will need the new lead bend. You off course will of course have to also have some plumbing experience or be working under the direction of a licensed master plumber. Install a new lead bend with lead and oakum is not something that everyone will be able to do and requires a good plumbing skill set.
Now that you have everything ready you will want to place the smelting pot on top of your burner and then you will want to add some lead into the pot. Once the lead is into the pot you will want to turn on the burner flame and start to heat up the lead so it can become liquid. While the lead is heating up you can take your new lead bend and place it into your cast iron pipe and then you will want to pack the joint with oakum. Now that the joint is packed with oakum you will want to wrap the runner rope around the joint so you will be ready to pour the lead into the joint with your lead ladle. Make sure that the runner rope is against the opening of the cast iron pipe where the end of the lead bend is to make sure when you pour the lead you will have a nice and clean lead joint.
After the runner rope is all ready to go around the oakum packed joint you can take your lead ladle and scoop some lead from your smelting pot and slowly pour it into the joint. The runner rope should prevent any lead from pouring out the sides or the bottom of the joint. You will want to pour the hot lead into the joint until completely full. Allow the lead to cool off for about 45 - 60 seconds and then you will want to remove the runner rope and you will notice that the lead on the joint will now be held in place and the lead bend will now be installed into the cast iron drain pipe.
You know will want to take your lead packing tool and pack the lead into the joint. Take your lead packing tool and tap one area at a time, little by little around the entire lead joint. You will want to work the lead into the joint about 1/4" - 3/8" of an inch. Packing the lead into the oakum joint will complete the seal and prevent the joint from leaking. Once you have packed all the lead into the joint you can now star to clean up all your lead tools and put them away for the next time you need to install a new lead bend.