1. Buy at least 10-15 stocks across various industries (or buy an index fund)One of the quickest ways to build a diversified portfolio is to invest in several stocks. A good rule of thumb is to own at least 10 to 15 different companies.However, it's important that they also be from a variety of industries. While it might be tempting to purchase shares of a dozen well-known tech giants and call it a day, that's not proper diversification. If tech spending takes a hit due to an economic slowdown or new government regulations, all those companies' shares could decline in unison. Because of that, investors should make sure they spread their investment dollars around several industries.One quick way to do that for those who don't have the time to research stocks is to buy an index fund. For exa