Gold is one of the most ductile and malleable metals, meaning that it is quite easy to reshape or stretch. It is thus quite possible to resize your wedding band by stretching it, although this option may not yield the most aesthetically pleasing results. Generally, having a qualified jeweller resize your ring will give you seamless results, whereas resizing it yourself by stretching it may leave unsightly lumps or malformations. It also means that you need to have the tools for stretching gold rings available in your home, which not everyone does.
Check that your ring fulfils the criteria for resizing via stretching. Ideally, the ring should be a plain, solid gold band. "Boring" is the term of choice when it comes to stretchable rings. If you stretch an ornate ring, it may have a detrimental aesthetic effect on the ring. Stretching an ornate ring may distort the design of the ring.
Size your finger using your finger sizer. The finger sizer comes with a variety of metal rings of different sizes on a hoop. Slip your finger into each ring until you've found the correct size. You may have to relax your finger by bending it slightly in order to find the correct size --- if you keep your finger stiff, the ring will have trouble slipping onto it.
Place the ring on the mandrel over the marked numerical size that corresponds to the size of your finger, as determined in the previous step. Pull the handle of the mandrel. The mandrel will expand outward, stretching your ring with it.