A process combining creativity, technical know-how, psychology and marketing savvy.  The web is let down when compared to more mature industries by the ease of entry into "Web design" and the lack of forced accreditations and compliance in its suppliers.  This is experienced at its worst at the small business end, where it is common enough to find people technically skilled in site creation (as the toolsets and template designs available now make entry to the market so easy) but who haven't the slightest notion of building for marketability.  

You wouldn't drive your car to a garage where the mechanics approach you - hammer in hand - before discussing your problems.  Yet small businesses do exactly this with their site design.

There are 63 billion web pages out there.  Bank on the basis that your site will not be found, and if it is, that a surfer will have a choice of maybe 10 other sites to go to if your site fails to make it clear what you offer and lead them to take action with you.  

This implies an up-front research process with your client to really get under the skin of their idea.  More times than not with a startup, this may be the first time the entrepreneur has had a hard, objectivce light shone on their "baby" and you will often emerge with questions and suggestions for the overall proposition, just through the process of forcing them to step through a typical customer engagement.  Sometimes it also has to be said you conclude there is no business underlying the enthusiasm and it is a test of your own scruples to come out and say so - or at least identify your key concerns.  DON'T just implement their ideas as-is and walk away with their money. It is far more valuable to both sides to creatre a relationship that will have legs, than perform a one-off piece of work that damages the reputation of both parties.