Shrinking most kinds of fabrics is a simple enough process. Fabrics like wool present some difficulties, but most jujitsu gi's (uniforms) will be made from cotton or polyester, which are both fabrics that shrink with relative ease. Just keep in mind while shrinking clothing that it is an imperfect science, and a slightly large jujitsu gi will be a much better fit than a slightly small jujitsu gi.
Things You'll Need
Read the label before you do anything. Find out what kind of fabric you have, because the process for shrinking clothing depends on the fabric the item is made of.
Wash the gi in hot water if it's made from cotton. If the gi is made from polyester, however, you will want to wash it in cold water instead.
Regardless of whether the gi is cotton or polyester, you will want to set the dryer to high to dry the gi. This is where the shrinking, if any, will happen.
Check periodically on the gi. It is entirely possible to overshrink your gi, which would be even worse than a too-large gi.
If shrinking in this way doesn't work, there is always the option to take the gi to a seamstress or a tailor and have it adjusted.