How do you invest in your 20s?

It is usually best to invest the money as you get it  for a number of reasons: First, it helps you to reduce the chance of investing all your money at the "wrong" time (ie. right before a market drop). This is also known as "dollar cost averaging". Second, by automating your investments every month (or paycheck, etc.), you ensure that the money goes in...automatically, every month, without fail. When cash sits idle in people's bank accounts, it will tend to find another purpose other than investing. And finally, if you automate it regularly, it's just one less thing to worry about. 

I would add that if you do decide to invest on a regular basis that you do so though your 401(k) at work or though non-transaction fee mutual funds and ETFs at a discount broker.  This will allow you to invest and compound your wealth over time without having the drag of multiple trading fees each time you put money to work in the market.  Being conscious of fees eating into your profits when you are starting out is a key consideration for keeping more of your money.