Your priorities will determine the procedure you will follow.  If COST is your number one and only concern, purchasing from one of the mega stores may work for you.  You measure, you select, you install.  You may not need to use a consultant if cost is your only priority.  If any of the other items are of concern to you, using a consultant will give you the best overall value.  A consultant can help you make the best decisions, work within your budget, give you unique design options, and get the best quality for your money.