We are interested in purchasing a piece of land from a private sellers. The seller has an attorney. How do we start the process?

Contact a good real estate attorney in your area and ask him to represent your interests in the transaction.  Not all attorneys are "real estate" attorneys.  Ask your friends, neighbors, co-workers, etc and find someone who has bought or sold real estate lately (refinanced their home perhaps) and get a good recommendation for an attorney.  He will be able to give you guidance in negotiating the price; he should conduct a title examination or have that done, and cause a policy of title insurance to be issued to protect your investment.