Image source: The Motley Fool With an eye on building wealth over the long term, I think owning shares in blue-chip companies makes sense for me. I use my Stocks and Shares ISA to benefit from both the growth and income potential of some of the country’s leading companies. If I had a spare £20,000 to invest in my ISA right now, here is how I would go about it. Set an investment strategy Different investors each have their own objectives, risk tolerance, and investing style. So my first move would be to decide what I wanted to try and achieve. I am a long-term investor. But what would be my timeframe here? Investing on the basis of an annual income target three or five years from now could lead me to different choice versus being happy to park £20,000 in a Stocks and Shares ISA today and leaving it there for decades. Growth or income Next I would consider what my balance should be between growth and income shares. I like to compound dividends and holding a Stocks and Shares ISA with £20,000 of capital in it could be a good platform for that. So I would likely tilt my ISA towards income shares, with perhaps a quarter or fifth of my allocation being more growth-focussed. Income shares I have recently bought include Vodafone and Dunelm. On the growth front, I am presently eyeing shares including Alphabet, which if I had a spare £20,000 to invest in my ISA today, would certainly be on my shopping list. Apply risk management principles There are two basic elements to how I could use a Stocks and Shares ISA as part of my approach to building wealth. One is to invest in shares that give me a positive return on my investment, whether through share price growth, dividends, or a combination of both. The other crucial element is to avoid losing money on my investments. As billionaire investor Warren Buffett puts it, “rule number one: never lose money. Rule number two: never forget rule number one”. That is easier said than done, but I can at least try to aim for that outcome by applying some basic but important risk management principles. For example, like Buffett, I would diversify my investments across a range of companies. £20,000 is ample money to let me do this. Also like Buffett, I would stick to companies I felt I understood and could assess. Finding companies to buy In the long term, as Buffett has shown repeatedly, building wealth through investing is not just about buying into great businesses. It also relies on buying at an attractive price. That can take time and so I would be patient. I would not be in a rush to put the money in my ISA to work. Instead, I would spend time drawing up a list of businesses I felt had great characteristics, such as a competitive advantage in an industry with strong customer demand. I would then look out for opportunities to add those firms to my Stocks and Shares ISA at the right price.