What I did is that I looked into a couple of forward-thinking pension funds in my country. These are ones that only invest in index funds and have very low fees. From those, I picked three index funds that felt good - one covering Europe, one covering US and one covering the world - and I've been dumping money in them.

To me, this seemed like the most straightforward way to invest in index funds, keeping a comfortable risk level and not requiring any time from me at all. With the recent economic downturns, it has proven to be quite an OK strategy, as the market fluctuations between Europe, US and the world differ wildly, so the three funds sort of balance each other out.

Additionally, I max out the 401k-equivalent pension fund in my country.