How do you Weld with a cutting torch?

You turn on the Acetylene and ignite it, you start to add oxygen via the control valve. You will see the colour changes from yellow to blue. There are two shades of blue, one is slightly darker than the other. Once the inner blue cone is down to a nice roaring bright light you are more or less set up to weld. Get your Filler wire (welding rod) put on your welding goggles and lower the flame to the piece of metal you wish to weld. You will start to see the metal heat and become molten. The next part takes practice. Once the metal has turned molten immediately start to add the filler wire; keeping the torch at about 45º move along the metal adding the wire at a constant steady rate. Insufficient filler wire will result in either undercut or the pool dropping through. Too much wire or moving to fast will result in either a poor weld or no weld at all. Speed and angles do not need to be 100% correct find what is comfortable for you.

Remember Gas welding is Extremely Dangerous and you should take courses in it before attempting it.

I will add a couple of links for you to get a more detailed idea lower in the page under related, source links.