This can be as easy or lazy as you like. Don't want to do anything? Get a target date/balanced fund and eat the slightly larger expense ratio and realised income. Ok with rebalancing maybe once a year or so? Put X% into a stock index fund, Y% into a bond index fund, reinvestment on and look on your birthday/whatever day once a year to see if you need to rebalance.

You can of course spend more time, but if you don't like it, you don't have to.