There's plenty of good material on the top page of results on your favorite search engine.

It's more important to choose an allocation strategy and stick to it than it is to select the perfect allocation strategy.

Here's the hard part: determine your household's tolerance for risk. Are you willing to sit tight and not panic-sell if one or two of your funds decline by 40% in value? 20%? It's hard to know for sure how you will react until it happens. But, a decline like that means you should buy, not sell.

I mention "your household" because your spouse or other relatives might influence your risk tolerance. It doesn't matter if your anatomy is solid brass if your spouse insists on panic-selling. Part of the deal with risk tolerance is having conversations with family stakeholders about risk, to prepare for the inevitable downturn.

As the time grows nearer when you need to use the funds in your portfolio (retirement? college tuition?): reduce your stated risk tolerance once a year or so, and rebalance accordingly. This will help you lock in your gains even if something bad happens.

In the old, pre-junk-bond, days, "bonds" were considered lowest risk, "growth-and-income stocks" medium risk, and "growth stocks" highest risk. You can probably find index funds like those. Certainly you can find funds that invest in an index basket of low-risk bonds, or an index basket of dividend-paying (growth and income stocks).

"Unmanaged" is the key word to identify index funds. Vanguard, DFA, and others are the companies offering them.

One last thing: If you don't understand a fund's strategy, DO NOT INVEST IN THAT FUND. If the promises seem too good to be true, well....