Read the laws for your state regarding adverse possession. In Massachusetts, for example, a person must use a property continuously for 20 years and such usage must be open and notorious before you can claim it.
Occupy the property. By occupying the property, either by living on it or using it frequently, you are claiming ownership of it. Whether you plant your garden on the property and visit it everyday, park your car on it or pitch a tent on it, as long as your use of the property is obvious and notorious -- meaning the owner has a chance to throw you off of it -- it's legal.
Pay the property taxes during the time period for continuous use, and you have proof for a valid claim.
Contact your county clerk's office or deed registrar and fill out the form necessary to make your claim on the land. You may require a real estate lawyer to help you do this.
Apply for a title for the land. In some regions, all that is required is that you apply for the title, pay the property taxes and wait for the title to be sent to you.