Personal preference, I like to start at the bottom and work my way up. Heat rises and it is easier to be sure to fill the joint without having big blobs running down the pipe. Heat on one side on the fitting apply solder to the other side until it is almost ready to drip then move up to the top do the same. Don't over heat the fitting do it quick and wait just a few seconds gently wipe the solder to remove any drip, wait until it has cooled to warm to touch and wipe vigorously with a soft clean rag or scott blue shop towel to remove any excess flux. Use a solid solder made especially for copper pipe, usually silver, never lead, not any electrical or electronics solder.
Always wear eye protection the solder can splash, do not allow any one, especially children or pets to be around. Watch out for fires, never leave the area right away, keep a bucket of water handy and have a fire extinguisher handy just in case. I wouldn't use the extinguisher unless necessary because of the mess, when water works. You turned the water off so get a bucket first.
Take a piece of pipe out in the garage and clamp it to a work bench and practice first. Sand the outside of the pipe and the inside of the fitting first then apply the flux then heat and solder. Heat it back up and tear it apart and see how you did, the pipe and fitting should have a nice coating of solder. Make sure the pipe is all the way in the fitting or you could get solder into the inside of the pipe where the water sould be.
Some people talk about filling the pipe with solder, but I have tried and was never sucessful without the pipe not being all the way into the fitting. General rule of thumb is you will use as long a piece of solder as the pipe size, so 1/2" for 1/2" pipe. If you add solder until it starts to drip is another way. Practice first though. All water must be out of the pipe, it is almost impossible to solder with water in the pipe. If the pipe is wet, use a little plumber's bread to stop the steam and have a valve nearby open, you can not solder a closed pipe.