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in multiple phases 

 begin to evaluate the business opportunity and quickly conclude  

It's where you research and examine all aspects of the potential business to determine the requirements, resources required, and the impediments or risk factors.  At this stage, you've committed little money and minimal time.  If, after an initial assessment, the idea still looks good, you do a second, where you did deeper, do more research, get opinions from others, and develop a more detailed assessment of the opportunity.  Then, if the idea still looks good, you have to make a decision - are you able and willing to pursue the opportunity into the Execution phase, or do you put it on the shelf and wait until a better time?

 and If you jump over the Assessment stage and immediately begin work on building the business, you're potentially building on

start working up the idea to see what would it take to build that business.  There's nothing wrong with the first thing -- new ideas are exciting and they keep the creative, entrepreneurial juices flowing.  It's the second thing that can get you in trouble.  If the idea truly stinks, that is to say it has no chance of succeeding, and the vast majority do, you don't want to waste a lot of time and effort working on it, or even worse, you don't want to actually launch a business to pursue it!