Rings made of most metals can be re-sized although gold is probably the most popular because it is soft enough to be altered. Sterling silver rings can also be resized to fit your finger and this process is commonly done by a jeweler.
Note that all the tools that are used for this project are specialized equipment that jewelers typically have in their workshops. Jewelry-making courses usually have laboratories where you can find these tools so you can also seek the assistance of your local college to get hold of these materials.
To go down one size, cut the ring to remove a fraction of the material. You need to cut a part of the ring using a special saw and blade according to how small you want the ring resized. You want to leave a small allowance because sanding will further reduce the size of your ring.
Sand the cut edges. Smooth the edges with sandpaper before soldering it with a silver solder. It's important to note that silver and gold have different melting points, so you should use a solder that is adjusted to a temperature that will melt silver just enough for the ring to be reconnected.
To size up the ring, cut it at the back part (the area that is not normally exposed when you wear it). If the ring has settings–that is, if you're working on one that is embellished with stones–you want to cut it 180 degrees from the setting. Cut as cleanly and as close to a square as you can.
Gently open the gap using pliers. Open the ring just wide enough for the sandpaper and files to get through. Buff out the ring to create straight edges. Don't taper or round off the ends because you need a flat surface to make it easier to create a cleaner soldering.
Determine how much the ring needs to be sized up. Measure your finger with a ring gauge and gently reclose the ring to measure it against the gauge as well. Determine the difference between the measurements to find out how much silver stock you need to fill the gap to arrive at your actual ring size.
Cut the silver stock to an appropriate size. Mill and file the silver stock to follow the dimensions and contour of the ring.
Using a mallet, expand the ring to accommodate the cut silver stock. Pound the sides of the ring to widen the gap. Insert the silver stock into the gap and solder it to the ring using your silver solder.
Clean the ring with the acid bath. Remove dirt, oil and discoloration from soldering by dipping the ring in an acid bath.