Decide on the location of your standalone bar. Leave enough space for people to gather around it,
Choose material for your counter top. Granite, Formica, tiles and wood are all viable options for building a counter top. The surface of the counter probably will get wet often.
Install your sink and faucet, if you are adding one. You might need to consult a plumber to do this unless you have prior plumbing experience.
Install your counter top on the top of your cabinets at your chosen height. Add three or more bar stools facing the bar.
Stock your bar with glasses, different types of alcohol and snacks -- such as mixed nuts, chips or crackers. Add bottle openers, shakers, a blender and other accessories.