Probably the most fundamental tactical question is: how do you find which stocks to buy?
You can use any one of a number of stock screening tools on the Internet. Many online brokerages have their own tools that allow you to do this as well. However, whichever one you choose, the goal is the same: filter for just the S&P 500 stocks, then sort them in order of best 12-month price performance. Then pick the top 3, 5, or however many you want to purchase.
My preferred tool is to use Finviz.com. They have a free version which gives you everything you need:
In general though, I like to support the websites that I find really valuable (and Finviz is one) so that they can afford to stay in business, so I subscribe to their Elite service – which provides a number of additional advanced screening and portfolio management features I find useful. I encourage you to check them out and if you decide to subscribe, the above link is also an affiliate link which means I will receive a small fee as a thank-you for sending them a paying customer.